Help!!! Can I have a explanation so I can do them on my own? Instead of looking for answers

Answer:
Sqrt of 75 or about 8.66
Step-by-step explanation:
So we have the Pythagorean Theorem A^2+B^2=C^2 (only works for right triangles)
Then we plug in the numbers that we know into the equation.
4^2+X^2=9^2 then we solve for X