c# - calculator number formatting C#

Question

I am working on a calculator in C# and this project is going to end me. I just learned about classes and my extent of formatting decimals is basically using formatting codes ex: ("C") for currency.

Here is what the project asks for:

"The results of multipication should show no more decimal places than the result of the first number than in the second number. Example 55.5*89.68 = 4977.240" and then a few other restrictions on addition, subtraction, and so on.

Currently the way my program is coded everything looks fine accept for multiplication. I believe what is throwing of my answers is how I have coded my division method.

Here is the code starting from my operator buttons. I did not format my output until I got to Division because that was the only test data that did not run correctly. When I formatted Division it is now throwing off Multiplication. I am unsure how to make both Multiplication and Division happy.

    private void btnAdd_Click(object sender, System.EventArgs e)
    {
        calc.Add(displayValue);
        newValue = true;
        decimalEntered = false;
        displayValue = calc.CurrentValue;
        txtDisplay.Text = displayValue.ToString();
    }

    private void btnSubtract_Click(object sender, System.EventArgs e)
    {
        calc.Subtract(displayValue);
        newValue = true;
        decimalEntered = false;
        displayValue = calc.CurrentValue;
        txtDisplay.Text = displayValue.ToString();
    }

    private void btnMultiply_Click(object sender, System.EventArgs e)
    {
        calc.Multiply(displayValue);
        newValue = true;
        decimalEntered = false;
        displayValue = calc.CurrentValue;
        txtDisplay.Text = displayValue.ToString();
    }

    private void btnDivide_Click(object sender, System.EventArgs e)
    {
        calc.Divide(displayValue);
        newValue = true;
        decimalEntered = false;
        displayValue = calc.CurrentValue;
        displayValue = Math.Round(displayValue, 8);
        txtDisplay.Text = displayValue.ToString();
    }




    private void btnEquals_Click(object sender, System.EventArgs e)
    {
        try
        {
            if (newValue)
                calc.Equals();
            else
                calc.Equals(displayValue);
            displayValue = calc.CurrentValue;
            displayValue = Math.Round(displayValue, 8);
            txtDisplay.Text = displayValue.ToString();
            newValue = true;
            decimalEntered = false;
        }
        catch (DivideByZeroException)
        {
            displayValue = 0;
            txtDisplay.Text = "Cannot divide by zero.";
            newValue = true;
            decimalEntered = false;
        }
    }

}

}

I also should note I tried adding the following code into my division statement instead of the way I have it above, and this gave me errors which is why I went back to the way I have it above.

calc.Divide(displayValue);
        newValue = true;
        decimalEntered = false;
        displayValue = calc.CurrentValue;
        txtDisplay.Text = displayValue.ToString("f8");