4a) a - 4b = 6 + 2b
a = 6(b+1)
4b) To dealt with this kind of question, you have to convert the description into equation. It asked for the change of a, so we would better assume △a as the unknown. △a could be positive or negative to represent increment or decrement repectively. We start with the relation between a and b,
a = 6(b+1)
After b is decreased by 2, LHS becomes
a+△a
RHS becomes
6((b-2)+1)
Equate both sides gives
a+△a = 6(b-1)
6(b+1)+△a = 6(b-1) [∵ a=6(b+1)]
△a = 6(b-1) - 6(b+1)
△a = -12
So, a is decreases by 12
