I would say: as the hours spent studying per week increased, the grade point averages increased as well, so all of it is just increasing. I say this because it begins at 0, or almost nothing and works its way up. Hope this is helpful.
Subtracting a negative is the same as adding a positive; therefore -18--18 = -18+18 = 0. Since we know that x>-18, 0 is below anything we could get from this.
