Calculate the length of the third side.

If you are trying to find out the hypotenuse of a triangle, with the known sides make them exponent two. Then add the total values and the find the square root Ta dah!! Example:
One side: 3
Two side: 4
Unknown side: x

First step: (3 x 3) + (4 x 4)
Second step: 9 + 16 = 25
Third step: Find the square root of 25, 5. Square root is like two numbers that are the same that make up a number like 5 x 5 = 25