AP and CD are parallel. Therefore the segments AC, CR, DR and PD are in proportion:
[tex]\dfrac{CR}{DR}=\dfrac{AC}{PD}[/tex]
We have
CR = x
DR = 42
AC = 10
PD = 15
Substitute:
[tex]\dfrac{x}{42}=\dfrac{10}{15}[/tex] cross multiply
[tex]15x=(42)(10)[/tex]
[tex]15x=420[/tex] divide both sides by 15
[tex]x=\dfrac{420}{15}[/tex]
[tex]\boxed{x=28}[/tex]
