I WILL GIVE 20 POINTS TO THOSE WHO ANSWER THIS QUESTION RIGHT NOOOO SCAMS AND EXPLAIN WHY THAT IS THE ANSWER

Answer:
4
Step-by-step explanation:
You have to use Pythagorean Theory which states that a^2+b^2=c^2
It means that:
3^2+b^2=5^2
b^2=25-9
b = root of 16 (25-9=16)
b = 4
I hope that I helped :)