How do I create particles that flow negative to the bullet

hey guys,

I'm having a small issue with particles...

I want to shoot a bullet and when the bullet hits the wall, i want it to create particles that flow on the opposite direction of the bullet...

So if the bullet is going to the right .... the particles must spawn to the left...

You might check out image points.

The vector would be bullet.angle-180

Thanks pal! it worked out allright