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

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