Excel 08 - Finding the gradient from a graph

Using excel from MS office 08 for mac I have plotted a graph from specified coordinates.

What im trying to do it find out the equation for the graph line. My windows buddies are doing the same and they can easily get the equation with a few clicks, how do I do this on the mac vershion?

EG. im afer something like Y = MX + C

Thanks

XY Scatter Cahrt should give you that.