I forgot what you would call this, but basically 3x = 120. both of the angles are the same. So divide both sides of the equation by 3. 3x/3 = 120/3 and your answer is 40.
A way of undoing any operation, is using the inverse of the operation. So if you want to ‘undo’ addition you subtract, and if you want to ‘undo’ multiplication you divide.
