Hello there!
You need to use the Pythagorean theorem.
a² + b² = c²
8² + 10² = c²
64 + 100 = c²
164 = c²
c = √164
c = 2√41
Decimal is 12.8
Thus, the length of the hypotenuse is 12
Let me know if you have any misunderstanding about the answer!
As always, I am here to help!