Please explain the calculation of the marginal cost of labor equation for the following problem:

A firm faces a perfectly elastic demand for its product at a price of $6 per unit of output. The firm, however, faces an upward-sloping labor supply curve of E = 20w - 120, where E is the number of workers hired each hour and w is the hourly wage rate. Thus, the firm also faces an upward sloping marginal cost of labor curve. Each hour of labor produces 5 units of output.

What is this MCE (marginal cost of labor) curve that this firm faces?