First you need to find the balanced chemical formula.
CuCl₂+2NaNO₃→2NaCl+Cu(NO₃)₂
Then you can find out how much NaCl 31.0g of CuCl₂ should produce using stoichiometry. Divide 31.0g by the molar mass of CuCl₂ (134.446g/mol) to get 0.2306mol CuCl₂. Than multiply 0.2306mol CuCl₂ by 2 to get 0.4612mol NaCl. Than multiply 0.4612mol by the molar mass of NaCl (58.45g/mol) to get 26.95g of NaCl.
that means that 100% yield would give you 26.95g of NaCl so to find percent yield divide 21.2 by 26.95 to get 0.7867 which is 78.7% yield
Therefore the answer is 78.7% yield.
I hope this helps. Let me know if anything is unclear