You can use the pythagorean theorem.

Now lets write out what we know.
Shorter leg = x
Longer Leg = x + 7
Hypotenuse = x - 7

Now solve for x

x = 28
Since you know x = 28, which is the shorter leg, you can solve for the rest by just plugging in 28 where x is located. For instance, longer let is x + 7 so
28 + 7 = 35