We can simplify this equation using logaritmic rules. logx-logb = log(x/b)
than our equation becomes. log3((18x^3)/(2x)) = log3(144) or log3(9x^2) = log3(144)
for x= -4 we get that left side is equal to right side log3(9*(-4)^2) = log3(144)
but if you put that value of x=-4 in start you will get log3(18(-4)^3) which is log of negative value which is imposible. That is why x=-4 is extraneous solution.