AB = 7X -4, BC =4X +5 i need to find the AC

Answer 1

Missing information.

Are you talking about a triangle, if so
what kind of triangle?
Is it right-angled?
which side is the hypotenuse?
Or perhaps AB is a street in Paris?
etc.

Answer 2

If B is the midpoint of AC, then AB=BC
7x - 4=4x + 5
x = 3
AB=BC=17 and AC=34