读一读,连线。
①-How many pencils? - Three pencils.____ a.
②- How many books? - One book.____ b.
③- How many rubbers? - Two rubbers. ____ c.
已知:M=x3﹣3xy+2x+1,N=﹣3x+xy,求多项式3M+2N,并计算当x=﹣1,y= 15 时,3M+2N的值.