You will be looking at trig (manipulating right angles), not calc.
Sin, Cos, Tan.
Basically you will break down an angle into components.
In FPS games, you will usually only have the angle you are facing and have to break the angle down into components.
You are at X, and you have the angle, also known as the hypotenuse (/) but not the y (|) or the x (_) component. So you take the
y = sin(angle)*movedistance or hypotenuse in a top-down RPG.
x = cos(angle)*movedistance or hypotenuse in a top-down RPG.
I wrote a little demonstration tool a long time ago when I was learning this stuff, it makes a circle stay 100 pixels in hypotenuse length away from the center circle based on the angle relation of the center circle to the mouse. I can send it to you if you need to.