Answer:
x = 5
Explanation:
Pythagorean Theorem

side a = x
side b = x + 7
side c = x + 8
therefore:

This would result in:

Let's combine our like terms:

This looks like a mess, but now we need to take all the value from the right side and move them to the left side using inverse operations. This would result in:

Again, combine like terms:

Now we factor:
(x-5)(x+3) = 0
Solving gives us:
x - 5 = 0 or x = 5 and x+3 = 0 or x = -3
Since we cannot have a negative distance we can discard the -3.
We plug in the 5 for X making the following:
Side a = 5
Side b = 12
Side c = 13
Check our Work

25 + 144 = 169
169 = 169 is true so x is indeed 5.
Whew!