Can you explain this general accounting question using accurate calculation methods?

Baxter & Co. rents a postage machine under an agreement that includes a fixed monthly fee plus a charge for each envelope processed. In January, the company processed 8,000 envelopes and paid $290. In March, it processed 12,000 envelopes and paid $370. Using the high-low method, compute the following: The variable cost per envelope The monthly fixed fee The total cost if the company processes 10,000 envelopes