What is the length of the hypotenuse of a right triangle with legs of lengths 6 and 8?

Being a right-triangle, apply Pythagoras.

Hence

h^(2) = a^(2) + b^(2)

Substitute

h^(2) = 6^(2) + 8^(2)

h^(2) = 36 + 64

h^(2) = 100

Square root BOTH sides.

h = 10 (The length of the hypotenuse.