Or we can use cross product of vector P0P1¯\bar{P_0P_1}P0P1¯ and P0P¯\bar{P_0P}P0P¯:
\begin{align*} \bar{P_0 P_1} \times \bar{P_0P} &= (x_1 - x_0, y_1 - y_0, 0) \times (x - x_0, y - y_0, 0) \ &= (0, 0, (x_1 - x_0)(y - y_0) - (x - x_0)(y_1 - y_0)). \end{align*}
L(x,y)L(x,y)L(x,y) is the third component of the result.
Or we can use cross product of vector P0P1¯ and P0P¯:
\begin{align*} \bar{P_0 P_1} \times \bar{P_0P} &= (x_1 - x_0, y_1 - y_0, 0) \times (x - x_0, y - y_0, 0) \ &= (0, 0, (x_1 - x_0)(y - y_0) - (x - x_0)(y_1 - y_0)). \end{align*}
L(x,y) is the third component of the result.