a) [3×27-(3×12)+(6×7)]-[(6×24)+(3×15)-(2×8)]
= (81-36+42)-(144+45-16)= (45+42)-(189-16)= 87-173=- 86
b) [(26×24)-(6×40)]-[(39×40)+(12×30)-(5×120)]=( 624-240)-(1560-600)=384-960=-576
