The obvious way to group two of the terms is to spot that 5b is common to 20b and 5ab, so let's start there: 5b(4-a). 2a^2 and 8a also factor into 2a(a-4). Apart from the sign the contents of the brackets on each case are the same. We can reverse the sign by negating from the outside of the brackets 5b(4-a) is the same as -5b(a-4). So we have 2a(a-4)-5b(a-4)=(2a-5b)(a-4).