Price reduction from 7.00 to 6.00, revenue increase from 7.00 to 12.00, the increase in revenue of 5.00 is the marginal revenue, marginal cost of 3.00 (1.50x2), profit of 9.00
Price reduction to 5, revenue of 15.00, marginal revenue of 3, total marginal cost of 4.50 (1.50x3), profit of 10.50
Price reduction to 4.00, revenue of 16.00, marginal revenue of 1, total marginal cost of 6(1.50x4)
The fourth unit gives 1.00 marginal revenue which is less than the 1.50 per unit marginal cost so it's no longer advantageous to sell 4 units
The 3rd unit will give 3.00 marginal revenue which is higher than 1.50 per unit marginal cost and raise profit to a maximum level of 10.50
The price of 5.00 is the optimum price that will maximize profit