16. $[\mathbf{1 0}]$ Let $R$ be a semicircle with diameter $X Y$. A trapezoid $A B C D$ in which $A B$ is parallel to $C D$ is circumscribed about $R$ such that $A B$ contains $X Y$. If $A D=4, C D=5$, and $B C=6$, determine $A B$.