Answer:
3
Explanation:
Long story short it's 3, how do I know without calculating? because this is the most common example used when people start learning about the Pythagorean theorem. 3^2 + 4^2 = 5^2
But anyways here is how to solve it
The formula for these questions is a^2 + b^2 = c^2, otherwise known as the Pythagorean theorem. a and b represent the sides that are going vertically and horizontally, doesn't matter which is which. c is the line going down diagonally, also known as the hypotenuse.
So here
a^2 + b^2 = c^2, replace b or a doesn't matter with 4 and c with 5 specifically.
a^2 + 4^2 = 5^2, now just solve for a
a^2 + 16 = 25
a^2 = 9
a = 3