y=2.5x+100 is a linear (straight line) graph so you only need two points to plot it. The best points are the intercepts. Mark these on the axes as follows: x-intercept (y=0) is when 2.5x=-100, x=-40 corresponding to point (-40,0). y-intercept (x=0) is when y=100, corresponding to (0,100). Choose your scale on the axes to allow these two points to fit, mark and label them and then draw the line passing through both. That’s your graph. Showing the intercepts like this on your graph demonstrates that you understand how to draw linear graphs.
Note that this graph DOES NOT represent losing points for tardiness. The proper graph would be y=-2.5x+100 where x is the number of days. The y-intercept is still 100 but the x-intercept is 40, not -40.