With this function, you can put your entire logic into one long mathematical expression and then click on "EVALUATE" and get your answer. Since the calculator uses radians, you will need to include the number of degrees per radian (57.3) in your calculations. Here's an example of what to enter for a height of 24 km and a radius that you determined to be 240 km:
57.3*atan(24/240)

Enter an expression here, click on EVALUATE, and then round-off your answer to 1 significant digit:

Slope of Olympus Mons:

You may choose to just use this calculator without knowing any trigonometry. If you want a brief review on angles and sines, cosines, and tangents, here is a great web site that covers the basics in an understandable way.
Source for JavaScript Calculator