because these are both right triangles, you can use the pythagorean theorem:
a² + b² = c²
a and b are sides while c is the hypotenuse.
for #7, you have two sides and you need the hypotenuse, so you'll set it up like so:
3² + 4² = c² ... solve for c, simplify exponents
9 + 16 = c² ... add
25 = c² ... square root both sides
√25 = c
c = 5
so your missing side for #7 is 5.
#8 follows the same process. you have one side and the hypotenuse:
a² + 15² = 17²
a² + 225 = 289
a² = 64
a = 8
your missing side for #8 is 8!!