The figure is a right triangle so we can use the Pythagorean theorem to find the third side AKA the hypotenuse.
Pythagorean theorem = a² + b² = c² where A and B are the legs of the triangle and C is the hypotenuse.
39² + 80² = c²
169 + 6400 = c²
6569 = c²
Square root both sides to cancel out the squared variable.
√6569 = √(c²)
√6569 = c
The third side, the hypotenuse, is √6569 units long.
