Need word problem help.

[Dillinger4]

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?

[Dillinger4]

Ah ok i got it. All i have to do is substitute the given points. c = 7/20f + b --> 73 = 7/20(80) + b --> 73 = 28 + b --> 45 = b