Rearranging the formula c2=a2+b2 for b

The answer would be c-a=b, Basically you move everything that is not b to one side and leave b alone

I assume this is the Pythagorean Theorem, and you want use to rearrange the formula so we find the value of b (not c).
Since c² = a² + b², then we know that if we subtract a² from c², then we get b².
So are rearranged formula would be b² = c² - a².
Hope I helped!