Use the Pythagorean theorem to solve this.
The Pythagorean theorem says that:
If a and b are the legs of a right triangle, and c is the hypotenuse, then
a^2+b^2=c^2.
Substiute in the values that you know to the equation.
3^2+4^2=c^2
9+16=c^2
25=c^2
5=c
The length of the hypotenuse is 5 feet.
Hope this helps!