Do you know what you have to show?
So if I let W=X, then "clearly" W and X are isometric. So all that remains to show is that X (or W) is everywhere dense in X bar. So, if I let a sequence in X be x_n --> x in X bar. Then d (x_n,x) < epsilon. So we can make the distance between them as small as we want. So X is dense in X bar??
Oh just btw, you know how in the general proof for completion, we use the metric on the completion as lim n--> inf of d (x_n, y_n). Here we can just use the same metric right? How do we justify the use of the same metric for X bar and X?