The -15 in the slope means that they are spending 15 dollars per day, since slope is rise over run, or in our case, dollars spent per day. Dollars is y (rise), days is x (run). The slope is negative since they are spending money every day. Thus, the -15 is the amount of money they spend per day, and 150 is the amount they started with. That's C. For D., follow the x-axis to 7 and go across to the point on the y axis it coordinates with to find that after 7 days, one week, he has 45 left.