Question 82536
This is a straight application of the formula which you have given. The hypotenuse (represented by c) is the required side. The other two sides given are the a and b sides. It doesn't matter which you call a and which you call b.
{{{c^2=a^2+b^2}}}
{{{c^2=3^2+4^2}}}
{{{c^2=9+16}}}
{{{c^2=25}}}
Find the square root of each side
{{{c=sqrt(25)}}}
c = 5 feet