As log8(x(x+6)) = log8(5x+12) we can raise e by each side this getting x^2 + 6x = 5x + 12
x^2 + x - 12 = 0
(x+4)(x-3)=0
x=3, x=-4
However we can only do logs of positive number and when -4 is substituted into the equation it gives a negative number so the only true solution is x=3