4.3. In rectangle $A B C D$, point $E$ is marked on the extension of side $C D$ beyond point $D$. The bisector of angle $A B C$ intersects side $A D$ at point $K$, and the bisector of angle $A D E$ intersects the extension of side $A B$ at point $M$. Find $B C$, if $M K=10$ and $A B=7$.
Answer: $\sqrt{51} \approx 7.14$.