_AgNO3 + _
Cu → _
Cu(NO3)2 + -
Ag

2AgNO3 + Cu = Cu(NO3)2 + 2Ag

Explanation:

First you see which side has the most elements. Which is Cu(NO3)2 + Ag. But, both sides have the same elements? But, on the reactants side, there is 2 of NO3. On the products side there is only one.

Reactants:            Products:

Cu = 1                    Cu = 1

NO3 = 1                 NO3 = 2

Ag = 1                    Ag = 1

They are all equal, except for NO3. So on the reactants side, you add a two to make it even.

2AgNO3 + Cu = Cu(NO3)2 + Ag

Reactants:            Products:

Cu = 1                    Cu = 1

NO3 = 2                NO3 = 2

Ag = 2                   Ag = 1

Now, the NO3 is equal, But the Ag isn't. But, you can add a 2 on the products side so the whole equation becomes equal.

2AgNO3 + Cu = Cu(NO3)2 + 2Ag

Reactants:            Products:

Cu = 1                    Cu = 1

NO3 = 2                NO3 = 2

Ag = 2                   Ag = 2