these are right triangles...so we can use the pythagorean theorem
a^2 + b^2 = c^2....where a and b are the legs and c is the hypotenuse.
(a) 3^2 + 7^2 = c^2
9 + 49 = c^2
58 = c^2....here we take the sqrt of both sides, eliminating ^2
sqrt 58 = c
7.6 = c <== the missing side is 7.6 cm.
(b) 25^2 + 15^2 = c^2
625 + 225 = c^2
850 = c^2
sqrt 850 = c
29.2 = c <== the missing side is 29.2 cm