First, you will need to flip the values.
12x - (7-3x) = -8x - 30
12x - 7 + 3x = -8x - 30
Add like terms.
15x - 7 = -8x - 30
Next, add 7 to each side.
15x - 7 + 3x + 7 = -8x - 30 + 7
15x = -8x - 23
Now, add 8x to each side.
15x + 8x = -8x - 23 + 8x
23x = -23
Divide each side by 23.
x = -1