To find a missing side of a right triangle use the Pythagorean theorem A2+b2=c2 C is the hypotenuse (long side) Plug into the equation 11^2+b2=12^2 121+b2=144 So subtract 144-121 to get the missing side you get 23 Take the square root It will be 4.7 If they want it rounded X=5 If I had any mistakes in my answer I am sorry but everything should be correct