Answer
The complete equation for that reaction is:
$Zn(s) + 2HCl(aq) -- \gt H_2(g) + ZnCl_2$
Work Step by Step
$Zn(s)$ is a metal and $HCl$ is an acid. Therefore, this is the reaction between a solid metal and an acid, which will produce hydrogen gas $(H_2(g))$, and a salt.
$Zn(s) + HCl(aq) -- \gt H_2(g) + Salt$
The hydrogen gas only contains hydrogen (H), so the salt must be maded of $Zn$ and $Cl$.
$Zn$ normally has a charge of $+2$, and $Cl$: $-1$, so we would need 2 $Cl^-$ and 1 $Zn^{2+}$ to balance the charge: $ZnCl_2$
$Zn(s) + HCl(aq) -- \gt H_2(g) + ZnCl_2$
Now, we need to balance the equation. We can do that by putting a 2 before $HCl$:
$Zn(s) + 2HCl(aq) -- \gt H_2(g) + ZnCl_2$
