21.6 grams
The balanced equation for the reaction is
Cu + 2AgNO3 ==> 2Ag + Cu(NO3)2
So you can easily determine that for each mole of copper used, 2 moles of silver are produced. So let's lookup the atomic weights of copper and silver.
Atomic weight copper = 63.546
Atomic weight silver = 107.8682
Moles copper = 6.35 g / 63.546 g/mol = 0.099927611 mol
Since we'll produce 2 moles of silver per mole of copper used, we get
0.099927611 mol * 2 = 0.199855223 mol
And to get the mass, just multiply by the atomic weight of silver. So
0.199855223 * 107.8682 = 21.55802316 g
Finally, round to 3 significant figures, giving 21.6 g.
