How do I graph c=20t+30
since the graph is a straight line, all you need to do is find 2 points, then draw a line through them.
The easiest two points to use are usually when x=0 or y=0 (or, t and c in this case)
setting t=0, we get c = 20*0 + 30, so the point (0,30) is on the graph.
When c=0, we get 0 = 20t+30, so t = -3/2, and the point (-1.5,0) is on the graph.
Take it from there.