To complete this problem you have to use the Pythagorean Theorem. This is a^2=b^2+c^2.
a^2=5^2+2^2
a^2=25+4
a^2=29
Then, you find the square root of both sides. 29 isn’t a perfect square root, so the length of the third side in simplest radical form is the square root of 29.