If the chord joining the points P_1(x_1, y_1) and P_2(x_2, y_2) on the parabola y^2=12 x subtends a right angle at the vertex of the parabola, then…

If the chord joining the points $\displaystyle \mathrm{P}_1\left(x_1, y_1\right)$ and $\displaystyle \mathrm{P}_2\left(x_2, y_2\right)$ on the parabola $\displaystyle y^2=12 x$ subtends a right angle at the vertex of the parabola, then $\displaystyle x_1 x_2-y_1 y_2$ is equal to