1. 2x-8+6x+21 = 12
2x+6x = -1
8x = -1
x = -1/8
2. 20-12x-15x-10 = 8-6x
20-12x-15x-10+6x = 8
10-21x = 8
-21x = -2
x = 21/2
3. -10x-15+12x-6 = 2x-21
-10x-15+12x-6-2x = -21
-21-0x = -21
x = 0
4. 7x-21-4x-2 = 20-15x
7x-21-4x-2+15x = 20
18x-23 = 20
18x = -3
x= -3/18 = -1/6