we have a right triangle, then we can use triangle fractions
to find CD with the angle B we use tangent because
[tex]\tan (\alpha)=\frac{O}{A}[/tex]
where lapha is the reference angle(on this case B), O the opposite side from the triangle and A the adjacent side from the angle differente to the hypotenuse
[tex]\tan (B)=\frac{5}{BC}[/tex]
and we can solve for BC
[tex]BC=\frac{5}{\tan (B)}[/tex]
