Помогите букву а и б

A²-3a+1=7   a²-3a-6=0   a²=6+3a   a)  2a²-6a+2=12+6a-6a+2=14 б) a²(a²-3a+1)-3a(a²-3a+1)=a(a²-3a+1)(a-3)=a(6+3a-3a+1)(a-3)=7(a²-3a)= 7(6+3a-3a)=42