Give a necessary and sufficient condition for the oriented area of the triangle $\displaystyle (0,0), (x_1, y_1), (x_2,y_2) $ to be positive?

Do we have to incorporate the equation $\displaystyle y_{1}x - x_{1}y = 0 $? What if you had $\displaystyle (x_1, y_1), (x_2,y_2), (x_3,y_3) $?