Im having a hard to figuring out how to set these up:

Regrind, INC, used typewriters platens. The variable cost to regrind each platen is $1.40. The total cost to regrind 70 platens is $400. Find the linear cost function to regrind platens. If regrind platens sell for $9.40 each, how many must be reground and sold to break even?

----------------------------------------

A rug is fit into a room so that a border of even width is left on all four sides. If the room is 12 feet by 15 feet and the area of the rug is 108 square feet, how wide will the border be?

Thanks