A company gives each new salesperson a commission of $300 for the sale of a new car. The sales person will receive a $100 increase for each additional car that person sells that week, so the person gets $400 for the sale next week. Which equation represents the number of cars a salesperson must sell to earn $4200 in commissions in a week? Recall that for an arithmetic sequence, a_1 + d(n-1) is equivalent to a_n. (The underscore is the exponent below the number.)