A charge per unit length $+$ is uniformly distributed along the positive $y-$axis from $y=0$ to $y=+a.$ A charge per unit length $-$ is uniformly distributed along the negative $y-$axis from $y=0$ to $y=-a.$ Write an expression for the electric field $($magnitude and direction$)$ at a point on the x $-$axis a distance x from the origin.