To solve for x, you can use the Pythagorean theorem. The Pythagorean theorem is a squared+b squared=c squared, when a and b are the feet and c is the hypotenuse.
I'm going to put in the values I already know: 40(40)+9(9)=x(x) 1600+81=x(x) 1681=x(x) 41=x
