The goal is to seperate (or cancel) out variable (x) from the other numbers.
1st Step: Since the '3' is being mutiplied with the x, it is best to deal with the constant, which is the negative 4. To cancel out a negative, you have to do the opposite of subtraction, which is addition.
So the negative 4 is canceled by adding a positive 4 to it, which results in a zero.
Everything that is done on one side must be done on the other so the 4 is also added to the 11.
2nd Step: Now only the 3x is left and the 3 must be seperated from the X. because 3x is multiplication you must do the opposite which is division. Dividing 3x by 3 results in the x being by itself because 3 cancels out.
Again, what you do on what side must be done on the other so you also divide the 15 by 3, which is 5.