combine like terms:
- -21a 16a
a -5a
b 5a
c 37a
d -37a
- 5 (x 10) x
a 6x 15
b 5x 15
c 6x 50
d 4x - 50
b 5a
c 37a
d -37a
b 5x 15
c 6x 50
d 4x - 50