-3x + 7x = -12 equals 
x = -3.
First, simplify -3x + 7x to 4x. / Your problem should look like: 4x = -12.
Second, divide both sides by 4. / Your problem should look like: x = 

 . 
Third, simplify 

 to 3. / Your problem should look like: x = -3, which is your answer.