Im a little confused on how to come up with the linear cost function here:

Regrind, INC, regrinds typewriter 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 ?