Question

find the length of the hypotenuse of a triangle with legs of 5 ft and 12 ft?
11 ft
13 ft
17 ft
60 ft

Answer 1

it's one of the "Pythagorean triples"

right triangles with integer sides

like 3-4-5

remember ... a^2 + b^2 = c^2

Answer 2

there are a few basic integer-sided right triangles you would do well to learn. These start with

3-4-5
5-12-13
8-15-17
7-24-25

and all multiples of these, such as
6-8-10, 10-40-50, ...

However, to solve the problem in general, the hypotenuse is

c^2 = a^2+b^2
In your case, we have
c^2 = 5^2 + 12^2 = 25+144 = 169 = 13^2
so, c = 13