$\begin{align}  & \text{Area of Square is }{{l}^{2}} \\ & {{l}^{2}}=144sq.cm \\ & l=12cm \\ & \text{All the sides are equal in a square} \\ & A{{D}^{2}}=A{{C}^{2}}+C{{D}^{2}} \\ & A{{D}^{2}}={{12}^{2}}+{{12}^{2}} \\ & A{{D}^{2}}=288 \\ & AD=\sqrt{288}=12\sqrt{2}cm \\ & \text{The length of the diagonal is }12\sqrt{2}cm \\\end{align}$