The answer is C=10
This is the Pythagorean theorem, it solves for the hypontunuse. Basically it tells you the missing sides length.
It’s really simple once you get the hang of it. The formula is
A^2+b^2=c^2
Plug in the side lengths you have (6 and 8)
6^2+8^2=c^2
36+64=c^2
100=c^2
DO NOT PUT 100 AS ANSWER
You then take the square root of 100, to get 10^2