3. A, B, and $\mathrm{C}$ are collinear points with $\mathrm{B}$ being the midpoint of $\overline{A C}$. If $A B=2 x+1$ and $B C=3 x-4$, then find $A C$.

Step 1 ：Given that B is the midpoint of AC, it means that AB = BC.

Step 2 ：Set up the equation \(2x + 1 = 3x - 4\) to solve for x.

Step 3 ：Solving the equation gives x = 5.

Step 4 ：Substitute x = 5 into either AB = 2x + 1 or BC = 3x - 4 to find the length of AB or BC. This gives AB = BC = 11.

Step 5 ：Since AB = BC, the length of AC would be 2 * AB or 2 * BC, which gives AC = 22.

Step 6 ：Final Answer: The length of AC is \(\boxed{22}\).