To solve this you can make a proportion where the matching sides will correspond to each other. It should look something like this:
[tex]\frac{AC}{PD} =\frac{CR}{DR}[/tex]
[tex]\frac{10}{15} =\frac{x}{42}[/tex]
Now you must cross multiply
10*42 = 15*x
420= 15x
To isolate x divide 15 to both sides
420/15 = 15x/15
28 = x
Hope this helped!
~Just a girl in love with Shawn Mendes
