Since it is a right triangle we Can apply the Pythagorean theorem:
c^2 = a^2 + b^2
Where:
c = hypotenuse (longest side) = x
A & b = the other 2 legs of the triangle (80 and 39)
Replacing:
x^2 = 80^2 + 39^2
Solve for c:
x^2 = 6400+ 1521
x ^2 = 7921
x = √7921
x = 89