How do I find the slack time of A? http://pic20.picturetrail.com/VOL1370/5671323/24520837/410059420.jpg?

Draw the critical activities nose-to-tail. See when A must be completed so it doesn't delay the project, then subtract from that the length of A.

The critical path is B-D-E-G. The starting day of each of these is 1, 7, 17, 21, respectively. In order to have A complete by day 17, it must start no later than day 14. Thus, there are 13 days of slack time associated with task A.