Since it decreases by 36 each year, the change would be 36+36+36+36=36*4=144
For a, we can plug in values for t, so 2000-(200*1)=1800 for the first year, 2000-(200*2)=1600 - do you see the pattern here? It subtracts by 200 every year, so 1600-200=1400 for the third year and 1400-200=1200 for the fourth year.
