You need to use the Pythagorean Theorem for this: c squared = a squared + b squared. C is the hypotenuse, or the side not making a right angle. In this case, that is your missing side. We plug it in.
C squared = 10 squared + 9 squared.
C squared = 100 + 81.
C squared = 181
Now, to get c squared to just c, we find it’s square root.
The square root of c squared is c, while the square root of 181 is 13.6 approximately.
