In order to do this equation, you need to know that d dot c= |d||c|cos(theta) where theta is the angle between them. I started to write "to find ID X CI" you would have to know what 'ID' and 'CI' mean" when it hit me that youmean|D X C|, thelengthof DXC. Of course, to do that you have to know that that length is given by |D||C| sin(theta).