A sidewalk vendor knows that selling 80 franks costs a total of $73 and selling 100 franks costs a total of $80. Assume that the total cost c of the franks is linearly related to the number of f franks sold.

Now i am supposed to write an equation relating the total cost of the franks to the number of franks sold.

I am using the equation y = mx + b and the two points(if this was graphed) (80,73)(100,80)

Now to get the slope im using y2 - y1 = m(x2 - x1) which gives me 7/20.

So so far my equation is c = 7/20f + b but how do i get b?