Answer:
3
Step-by-step explanation:
Pythagorean Theorem: a²+b²=c²
You can fill in the numbers you know: a²+4²=5²
(Hypotenuse must be side c)
Square those numbers and you have: a²+16=25
Subtract 16 from both sides: a²=9
Square root both sides and you get: a=3
