First, realize that x is in thousands of dollars. Thus, evaluate N for $10,000 (i.e., x = 10) and for $11,000 (i.e., x = 11). Since the range of x is provided, which when translated says N works for advertising budgets of $5,000 to $30,000, these evaluations for N will work. What do you get for N in each case? Take their difference. I will add, my answer was 39, not 40. In other words, the company will sell 39 more units when the advertising spending is increased from $10,000 to $11,000.