To find the missing side use Pythagorean theorem (a^2+b^2=c^2)
6^2 + 8^2 = c^2
36 + 64 = c^2
Add 36 and 64
100 = c^2
Square root both side to get rid of the square on the letter c and you have to do both side because what you do to one side you must do to the other.
C=10
