The side length is 16
So you know this triangle is a right triangle because it has the marking so you know pythagorean theorem a^2+b^2=c^2 will work c is the hypothesis and we are give the hypotenuse which is 20 we also have one side that is 12 so you plug that in to the formula 20 has to be in place of c but it doesn’t matter if 12 is a or b so once plug into the formula you have 12^2+b^2=20^2 we are trying to find b so you wanna simplify the equation first so 144+b^2=400 now you wanna get b by itself and isolate the variable to do that subtract 144 from both sides you’ll get b^2 =256 now to get b alone square root 256 to cancel out that exponent and you’ll get 16 so b=16
