"The manager of a factory finds that it costs $2200 to produce 100 chairs in one day, and $4800 to produce 300 chairs in one day. Find the slope of the cost function."

Can someone please explain to me how to do this one? I tried to put it into slope-intercept form, but couldn't do it. And my textbook doesn't have an example or even talk about how to do these. I assume I'm missing something massively obvious? Any help is appreciated!