Please I need help I’ve be stuck on this for sooo long I need the correct answer people keep giving me the wrong one

pythagorean theorem:

there are 2 sides and a hypotenuse, a and b represent the sides and c is the hypotenuse, the hypotenuse is always the longest side to put it simply, now all you have to do is substitute, side a is missing, so we are solving for that missing side, side b is 9, and side c, the hypotenuse, is 12

now simplify,

now since we are solving for the a side, we must isolate a

the 81's cancel out so it's just 144-81 which is 63

a still must be isolated, so we have to take the square root of both sides, that in turn cancels out the square on the a

the equation can no longer be simplified so the answer is