Answer:
The question is missing the amount of output units that each additional unit of labor generates, but we can calculate how many units each additional unit of labor should produce in order to maximize profit.
In order for a firm to maximize its profit, the marginal revenue product (MRP) = marginal cost (MC).
MRP = output units per additional unit of labor x price per unit = U x $9
MC = $700
U x $9 = $700
U = $700 / $9 = 77.78, so we round up to 78 units
In order to maximize profit, each additional unit of labor must generate 78 additional units of output.