Given below is unbalanced chemical equation,

$\mathit{a}\mathit{ }\mathit{AgN}{O}_3+ \mathit{b} \mathit{Cu}\to \mathit{c} \mathit{Cu}(N{O}_3{)}_2+\mathit{ }\mathit{d}\mathit{ }Ag$

What is the value of 'a' in the above given chemical equation after balancing the chemical equation?

In the given reaction, $N{O}_3$ is not balanced. Hence, multiply by 2 to $\mathit{AgN}{O}_3$
$2\mathit{AgN}{O}_3+\mathit{Cu}\to \mathit{Cu}(N{O}_3{)}_2+\mathit{Ag}$
Multiply by 2 to Ag on the product side to balance the Ag element,
$2\mathit{AgN}{O}_3+\mathit{Cu}\to \mathit{Cu}(N{O}_3{)}_2+2\mathit{Ag}$
Balancing both the sides with equal moles reactant and product.
 

Hence, the balanced chemical reaction equation is,

$2\mathit{AgN}{O}_3+\mathit{Cu}\to \mathit{Cu}(N{O}_3{)}_2+2\mathit{Ag}$