The Pythagorean Theorum formula is a(squared) + b(squared)= c(squared). If you plug in the numbers, the equation is 39(squared) + 80(squared)= c(squared). That simplifies to 1521+6400=c(squared). Adding results in 7921=c(squared). Finally, when you take the square root of both sides, you are left with 89=c (aka the hypotenuse)
If you are looking for the surface area of the cube, it is 169. The equation for the surface area of a square is base times height. This would mean it’s 13x13 since all the sides are the same. 13x13=169
