The equation that could be one of the equations in the system can be obtained by simply plugging in the value of the point or solution that both equations share.
The only equation where the the solution can be of P(0,0) would be y = -3x
Y = -3x
0 = -3(0)
0 = 0.
Thus y = -3x has a possible solution of P(0,0) and is one of the equations of the system.