Is there a picture to this?
According to the fundemintal counting princable the answer is 18.
Check the picture below.
notice the solid circle at -4 and 3, since it includes those points.
also notice the dashed line, though the lines keeps on going those ways, is not included in the piece-wise, since it's not part fo the domain or constraints.
A - 3b = 4
a = b -2
(b - 2) - 3b = 4
b - 2 - 3b = 4
-2b = 4 + 2
-2b = 6
b = 6/-2
b = -3
a = b - 2
a = -3 -2
a = -5
to check: a = -5 ; b = -3 ⇒ (-5,-3)
a - 3b = 4
-5 - 3(-3) = 4
-5 + 9 = 4
4 = 4