a,-12.(x-5)+7.(3-x)=5
=>-12x-(-60)+21+7x=5
=>-12x+60+21+7x=5
=>-12x+7x=5-60-21
=>-5x=-76
=>x=-76:-5
=>x=15,2
b,30.(x+2)-6.(x-5)-24.x=100
=>30.x+60-(6.x-30)-24.x=100
=>30.x+60-6.x+30-24x=100
=>30.x-6.x-24.x=100-60-30
=>0x=10(vô lý)
Vậy không có x thỏa mãn