to solve this you will you the Pythagorean Theorem.
the equation for the Pythagorean Theorem is a²+b²=c².
fill in the information you have:
a²+3²=5²
now solve for a:
a²+9=25
-9 -9
a²=16
√a²=√16
a=4
The length of the side a of the right triangle is 4.
Hope I helped...
