Simple 2d collision response

Hi,

I am having some problems in calculating the collision response of the following situation:

I have a rectangle moving at a certain dx and dy velocity inside an octagon (It’s a dice game). When it collides with an edge of the octagon I need to calculate the new dx and dy values.

The top and bottom edge is offcourse: dy=-dy
And left and right edge: dx=-dx

But what about the other edges ??? :-[