[474+3(4a-12)]:19=30
474+3(4a-12)=30×19
474+3(4a-12)=570
3(4a-12)=474+570
3(4a-12)=1044
4a-12=1044:3
4a-12=348
4a=348+12
4a=360
a=360:4
a=90
