24+12=5x-3x
2x=36
x=18
验算：左边=3*18+24=78,右边=5*18-12=78,左边=右边
5x-4=45/3
5x-4=15
5x=19
x=3.8
验算：左边=3(5*3.8-4)=3*(19-4)=45,右边=45.左边=右边