72X2.09 - 1.8X73.6 = 72X2.09 - 18X7.36 = 18X(4X2.09 - 7.36)=18X(8.36 - 7.36)= 18
72×2.09-1.8×73.6=1.8×40×2.09-1.8×73.6=1.8×83.6-1.8×73.6=1.8×(83.6-73.6)=1.8×10=18