You isolate the x term basically.
First you bring 4x from the right hand side to the left hand side and so the equation would be: 9x - 4x + 42 = -13
Then you bring 42 from the left hand side to the right hand side so the equation would look something like this: 9x - 4x = -13 - 42
Next subtract the x terms and the numbers on the right hand side too so you get: 5x = -55
Remove the coefficient from x and bring it to the right hand side with the operation as division. So you get: x = -55/5
Solve the division and you get: x = -11
:D
