use pythagorean's theorem here, as that is a right triangle. the theorem states that a^2+b^2=c^2, with a and b as the legs and c as the hypotenuse. you have the hypotenuse and a leg, so plug those values in to solve. 2^2+b^2=8^2, so 4+b^2=64. subtract four from both sides to get b^2=60. square root both sides to get b=sqrt(60), which is 2root15, or about 7.75
