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