You would solve this problem by using Pythagorean Theorem. This theorem states a^2+b^2=c^2. C is ALWAYS the hypotenuse. We would have to solve for the smaller triangle first. It would look something like this.
3^2+b^2=5^2
9+b^2=25
b^2=16
b=4
Therefore, the missing side to the small triangle is 4. Now, we can solve for the hypotenuse of the bigger triangle,
4^2+7^2=c^2
16+49=c^2
65=c^2
Side z is approximately equal to 8.