You have to use the formula. Its quite simple. Its a^2+b^2=c^2.
But since you are trying to find one of the legs than you would do it backwards. Since c is the hypotenuse, than youd do 10^2(which equals 100) subtracted by 8^2(which equals 64). This should give you 46. Than you would find the square root of 64 and that is your answer.