How do I calculate marginal benefit and marginal cost?

I'm having a difficult time understanding marginal benefit and marginal cost and need some help. Here's the problem: New printers cost $2,000 and the expected total dollar benefit of the printers is (1) $3,500, (2) $7,000, (3) $10,000, (4) $12,000, and (5) $13,500. How do I calculate the marginal benefit and marginal cost? Thanks in advance for any help!