a^2 + b^2 = c^2 (this is the formula for finding the hypotenuse of a right triangle)
All you have to do is plug the values into the formula:
6^2 + 8^2 = c^2
36 + 64 = c^2
100= c^2
Now find the square root of 100, which is 10. So, the missing measure is 10 inches.
