y-a = (3t²-18t+15)(x-0)
y = (3t²-18t+15)x +a
これが点(t, t³-9t²+15t-7)を通るので
t³-9t²+15t-7 = (3t²-18t+15)t +a
t³-9t²+15t-7 = 3t³-18t²+15t +a
-2t³+9t²-7 = a

出ましたが…