Will Tummy Tuck Get Rid of Old Horizontal Scar?

Thank you for your question. Based on your photos you appear to have abdomen sagging skin excess, several stretch marks and unaesthetic scar makes you a full tummy tuck candidate.  I would suggest to be treated by board certified plastic surgeon with experience in this procedure, to get the best desired results.
Respectfully,Dr. Emmanuel Mallol Cotes.-

It appears as though you will most likely be able to get rid of your scar with your tummy tuck.  I have had a lot of good experience removing old c-section scars with a tummy tuck.

Scars at or below the belly button may be removed by tummy tuck, but in my opinion your scar is too high for this, and a higher tummy tuck scar would be equally unsightly.
A tummy tuck with a bikini-line incision may help hide and flatten this scar somewhat due to the skin roll above it..

The new tummy tuck scar will be lower and should be designed to incorporate and hide a small bikini. However, with the degree of scarring, you should view typical abdominoplasty scars and see if they are acceptable to you prior to having the surgery done.

based on your pic I would guess no way, unless you want your scar to be 4 inches wide or at the level of your belly button. tread lightly here. good luck