Seeing that its a right triangle...
We can use pythagorean theorem.
8^2 + 4^2 = x^2
64 + 16 = x^2
80 = x^2
x = 8.94427191
What is this supposed to be? Is there any actual question?
When you have

you can solve the equation by multiplying both sides by 1/4. Then simplify.

Answer:
use mathaway or algebra calculator they help alot