1) Find out how many moles of Cl there is in that sample.
You are given all the information to find it. From the molarity formula we know that volume x molarity= #of moles, there for 0.071x0.2=0.0142 moles of Cl as well as Ag (but we don't need the moles of Ag obviously)
2) Now that you know how many moles of Cl you have you can find the # of grams of CL in the sample. 0.0142x35=0.49 we can round it to .5 to make our calculations easier.
3) Now all that's left is to find the percentage of Cl in the sample.
0.5g/5g=.1
.1x100%=10%
Hope this helps.