We have at the left a moltiplication, in fact is 4*x and 4*(-3)
4x-12 = 11x
Now, what happens?
We take all the number without the x and we move them at the right with the changed sign; and then we take all the numbers with the x and we move them at the left with changed sign
We get: 4x-11x = +12
-7x = 12
Now, the x can't be negative, so we change the sign to both terms