The graph is linear (a straight line) because there are no powers of x or y (other than power of 1). You only need two points so that you can connect them by a line. Put x=0 into the equation and you get y=0. That gives you the point (0,0) or the origin where the axes meet. Now put x=4 and y becomes 2.5*4=10, to give you the point (4,10) (the graph is a grid, 4 divisions (columns) to the right and 10 divisions (rows) straight up). Draw a line through (0,0) and (4,10) so it doesn't just join the points, but continues past them, and that's your graph.