This picture depicts a right triangle. When using a right triangle to find a certain side, you must use the pythagorean theorem which is
a^2 + b^2 = c^2
a and b are the legs and c the hypoteneuse
Plug in the numbers and solve
5^2 + 12^2 = c^2
25 + 144 = c^2
169 = c^2
Now square root both sides
Answer is x= 13
