I need help to a can get an A

See below:

Step-by-step explanation:

We can use the Pythagorean Theorum to find out the missing side because we have a 90 degree right angle and 2 sides already given to us.

The Pythagorean Theorem states that a^2 + b^2 = c^2

a in this case would be the shortest side,

b in this case would be the 2nd longest side,

c is the hypotenuse.

If we solve this, we get 8!

Cheers!