3. The first railway independently designed and constructed in our country, the "Jingzhang Railway," adopted a "human-shaped" design. Now, a section of the human-shaped road is to be built, as shown in the figure, where the length of $\mathrm{A} \rightarrow \mathrm{C}$ is 25 kilometers, the length of $\mathrm{C} \rightarrow \mathrm{D}$ is 36 kilometers, and the length of $\mathrm{A} \rightarrow \mathrm{B} \rightarrow \mathrm{D}$ is 43 kilometers. What is the length of $\mathrm{B} \rightarrow \mathrm{C}$? $\qquad$ kilometers.
$9$