Answer
See the picture below:
Work Step by Step
Assuming, that $\overline{AC}=\overline{AB}=\overline{BC}$
triangle ABC is an equilateral triangle. This means $\angle ABC = 60^{\circ}$. But $\angle ABC = 90^{\circ}$ because ABCD is a square.
Therefore $\overline{AC} \ne \overline {AB}$.
