2a+3b+6a+9b-8b+12b
=2a+6a+3b+9b-8b+12b
=(2+6)a+(3+9-8+12)b
=8a+16b
3pq+7pq+4pq+pq
=(3+7+4+1)pq
=15pq
7xy-8wy+5?
是7xy-8xy+5吧
=(7-8)xy+5
=-xy+5
=5-xy