In order to make this equation true you would have to add to it if you want w to equal 12. As it stands the equation would work out as follows...
7w = 87
w = 87/7
w = 12 3/7
As you can see 12 3/7 does not equal 12. The answer is 12 with 3 left over, so to make the equation true with w remaining 12, we have to take those 3 extras away by changing the problem...
7w + 3 = 87
7w = 87 - 3
7w = 84
w = 84/7 = 12