You see that -x and -5x both have an 'x' variable? This means they are 'like' terms. To get -x to the other side, we must add (if like terms are on opposite sides, do the opposite of what the sign says, e.g. - will be +)
12 = 20 - 4x
Now move 20 to the other side. Remember, do the opposite! Since 20 is positive, we must minus 20 to the other side.
-8 = -4x
To get x by itself, you will need to get rid of the coefficient of -4. -4x basically means -4 times x, so the opposite of that would be divide! Divide by -4
2 = x (Since both sides have a negative, they cancel out to give a positive) x = 2
