Ineed with the following equation:

a^6 + a(4-b) + 6b

where a = -2

The first thing you want to do is put the -2 where the letter a is. then you would want to next multiply -2 six times. then multiply the -2 into 4 and also into b which will give you so far: -64 +8 +2b+6b. Then what you would want to do next is to add all the like terms. which in this case would be the -64 and 8 together and then the 2b and 6b together too. which then you should have : -56+8b=0 and then you would want to add the -56 to the other side which then would give you 8b = 56 and then you would divde each side by 8 and finally you would get b=7. ( I hope that was helpful for you)