900a+30b+c=6....(1)
3600a+60b+c=7...(2)
10000a+100b+c=4....(3)
(2)-(1)
2700a+30b=1.....(4)
(3)-(1)
9100a+70b=-2...(5)
solving (4) and (5) then putting a, b into (1)
a=-13/8400, b=29/168, c=31/14
The graph is like a parabola with upward dome, it's head at upside and two legs downwards.