-4y+(-4Y)
6x
-3a + 6b
2a + (-7b) + [-9a -3b]
-15z+3z
-10y+2y
-9z+(-3z)
2a + 4b - (5a + 2b)
-8y
-7a + (-10b)
-12z
-2x+8x
10x-4x
