If a company manufactures 20 widgets it cost $405, the cost of manufacturing 500 is $1125. Assume the cost C depends linearly on the number x of widgets manufactured.

express the cost n terms of the number of widgets manufactured

You have 2 points on the graph, so you can just take the generic equation for a line:

y = mx + c

and work out what m and c must be.

What specifically are you having trouble with?

Which numbers do I plug where?

m is the gradient of the line, i.e.

m = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}

and c can be obtained by plugging in one of the points.
This can then be verified with the other point.

I got! Thank you