The problem is asking for you to solve for c.
The equation is a^2 + b^2 = c^2.
By ^2 I mean to the power of 2.
The triangle gives you a and b already so we have to plug it into the formula, which will give us 6^2 + 8^2 = c^2
This simplifies to 36 + 64 = c^2
Which is: 100 = c^2.
Then it simplifies to
c = 10 the final answer!