Because you know what A is equal to in terms of B, you can replace the A in the first equation with b-2, since we know that that also equals A. This makes the new equation b - 2 - 3b = 28. You can simplify this to -2b = 30. This makes B = -15. if you put this into the second equation now, -15 - 2 = A. This makes A = -17. I hope I could help to explain and answer this!
