> E1 := subs({x=-1, y=6}, P); E1 := 6 = a - b + c

> E2 := subs({x=1, y=4}, P); E2 := 4 = a + b + c

> E3 := subs({x=2, y=9}, P); E3 := 9 = 4 a + 2 b + c

> solve({E1, E2, E3}, {a,b,c}); {c = 3, b = -1, a = 2}