Parabola y2=12x has 4a=12, so a=3.
Parametric points: P1=(3t12,6t1), P2=(3t22,6t2).
Chord subtends right angle at vertex O(0,0): OP1⋅OP2=0.
9t12t22+36t1t2=0
⇒t1t2(t1t2+4)=0
⇒t1t2=−4.
x1x2−y1y2=9(t1t2)2−36(t1t2)
=9(16)−36(−4)=144+144=288.