Suppose that a firm rents labor services in a competitive labor market where the equilibrium wage ...

The graph should show a perfectly elastic (horizontal) supply curve at a wage of $15 per hour.  The labor supply curve is perfectly elastic because the labor market is "competitive."  The student should explain that in a competitive labor market there are many firms vying to rent labor services at the market wage.  If the firm of interest offers a wage below the market rate of $15 per hour, no worker in this skill category will be willing to work for the firm, as there are plenty of would be employers offering the market wage of $15 per hour.  Competition in the labor market means that the firm is a price taker in the labor market.