parts: A B C D
3a+6(b-8),
just using parts, combine(multiply) A and C, then A and D, B and C, then B and D. Obey +/- rules in multiplication, ie. if one integer is negative, product is negative. Two negative integers produce a negative product. ex. AC + AD + BC + BD.
Now, combine the actual parts of the original problem and you get:
3ab -(3a x 8) + 6b -(6 x 8)
Now simplify by multiplying what you can.
3ab - 24a + 6b -48.