you see that the term (x-8) is common (the greatest common factor) to both terms in the expression 6x(x-8)-5(x-8) so if we factor it out we have (x-8)(6x-5)
2. -18a^{5} -30a^{3 }+24a^{2 }the greatest common factor is 6a^{2 }in this case it's the term with the smallest exponent so we have 6a^{2}(-3a^{3}-5a+4) make sense?