Relay race. to start, you must run 20 yards to checkpoint a. you then must turn and run 12 yards to checkpoint b. last, you must turn and run back to the starting line. what is the distance from checkpoint b to the starting line?

Same thing here with Pythagorean's theorem, we are just going to rearrange it to find something else. Let's assume that from the start to check point B is A. Let's solve the theorem for A then.
A^2+B^2=C^2 (subtract B^2 from both sides)
A^2=C^2-B^2 (now take the square root of both sides) 
A=

Add in your numbers now and you get
A=
A= 16 yards