A local company is producing a product that will eventually be sold in the market for $20 per unit. The firm using 2 inputs, x and y, to produce but it is only able to select the amount of x, while y is held fixed. Find the amount of x that they should choose to use if they are going to maximize their profit. Assume that the cost of using x is $7 per unit, y will be fixed at 10 units, and the cost of using y is $3 per unit.