It seems that you have missed the necessary options for us to answer this question, so I had to look for it. Anyway, here is the answer. Since Jack is constructing the circumscribed circle of with just a compass and a straightedge, the step that he should take next in completing the construction is this: Construct the perpendicular bisectors of AB¯¯¯¯¯ and AC¯¯¯¯¯ by drawing a line through E and F and another through G and H.
