2.8×23.4+35.9×7.2
= (10-7.2)×23.4+35.9×7.2
= 10×23.4 - 7.2×23.4+35.9×7.2
= 234 + (-23.4+35.9)×7.2
= 234 + 12.5×7.2
= 234 + 12.5×8×0.9
= 234 + 100×0.9
= 234 + 90
= 324