a) 12 -x = 3x , we have to solve for x , it means we have to isolate x
So add x to both sides
12-x+x = 3x+x
12 = 4x
divide both sides by 4
3 = x
or
x=3
b) 4x-7 = 2x+3
subtract 2x from both sides
4x-2x-7 = 2x-2x +3
2x - 7 = 3
add 7 to both sides
2x - 7 + 7 = 3 + 7
2x = 10
divide both sides by 2
x=5
c) 3x-8 = x
Subtract x from both sides
3x-x-8 = x -x
2x - 8 = 0
Add 8 to both sides
2x - 8 + 8 = 8
2x = 8
Divide both sides by 2
x=4
d) 3 - 7x = 7 - 9x
Add 9x to both sides
3 - 7x + 9x = 7 - 9x + 9x
3+2x = 7
Subtract 3 from both sides
3 - 3 + 2x = 7 -3
2x = 4
Divide both sides by 2
x=2