The lengths of two side of a right triangle are given find the length of the third
side

Use the Pythagorean Theorem:  a² + b² = c².

Step-by-step explanation:

For any given right triangle the sum of the measures of the sides squared is equal to the hypotenuse squared.  The formula is call the Pythagorean Theorem and is written:  a² + b² = c², where 'a' and 'b' are the legs of the triangle and 'c' is the hypotenuse.  

Depending on which side of the triangle they give you, you can plug the other two side lengths into the equation to solve for the third.