4 + x = -8
4 + x - 4 = -8 - 4
x = -12
Know the reciprocal of the operation used then do the operation on each of the equation. Just like on the example, the operation of 4 (+4, when it's the first term, the positive sign will be hidden.) which is + (addition) will be just uhh -4. subtract 4 to each of the equation.
Sorry, I can't explain it well but I hope it help.