The vertical matchsticks on the scale all have the same weight. To balance the unbalanced scale you need to MOVE 1 matchstick. Where would you place it?
Left of balancing point: (3x1) + (1x1) + (1x1) + (1x1) = 6 units
Right of balancing point: (3x1) + (4x1) = 7 units
From the above we can see we need to move a matchstick right of the balancing point. This is done by placing the matchstick as shown in the solution.