This question can be solved with the pythagorean theorem.
The equation that we will be using is:
a^2 + b^2 = c^2
We know the first two sides (a and b)
So far we have:
5^2 + 5^2 = c^2
Lets simplify what we have so far:
25 + 25 = c^2
Simplify again
50 = c^2
Now take the square root or both sides:
~7 = c
The answer comes out to 7