If $6+9+12+15+18+21=\sum_{n=1}^{a}(b+c n)$ , Then $a=\ldots, b=\ldots$ , And $c=$

If $6+9+12+15+18+21=\sum_{n=1}^{a}(b+c n)$ , then $a=\ldots, b=\ldots$ , and $c=$