it cost pacific printing $23 to print 200 business cards and $35 to print 500 business cards. Let y be the cost of printing x cards. Question? What is the cost function if it is linear?

If you let y = cost of printing and x = number of cards, then you can say that you have two points (x, y).

So, you get:
(200, 23) and (500, 35)

You have two points, find the equation of the line passing through both points, having the form: y = mx + c where m is the gradient.

The way I do this is finding the gradient first using \frac{y_2-y_1}{x_2-x_1} = m

Then, I use the gradient with one point (either one) and make y the subject of formula:

\frac{y - (y-coordinate)}{x - (x-coordinate)} = m

Post what you get :)