use pythagorean thm. a^2 + b^2 = c^2
in this case c is 10 and either a or b is 8
so 8^2 + b^2 = 10^2
64 + b^2 = 100 subtract 64 from both sides
b^2 =36 then take the square root of both sides b=6
because it is a length then you don't consider the fact that b could be 6 or -6, it must be positive