Since the forces are directly opposing each other, we can subtract their magnitude to obtain the net force on the system.
Assume the positive direction is to the left:
[tex]f_{net} = f_{left} - f_{right} \\ f_{net} = 15 - 10 \\ f_{net} = + 5 \: newton[/tex]
5 Newtons to our assumed positive direction, which is to the left, so 5 newtons to the left is the answer.
C
