How do you do pythagorean theorem when you need to fine the a

Given that Pythagorean Theorum is
A^2 + B^2 = C^2, you need to do the opposite.
To find for A^2, you need to:
Subtract B^2 from C^2. This creates the equation of:
C^2 - B^2 = A^2. This works because A^2 + B^2 is C^2, so the opposite must work.
Now if you're looking for A and not A^2, you need to subtract B^2 from C^2 and then square root the difference.
I hope this helps!