$3$a$.$ Using Hess' Law, find the $H$ for the combustion of octane, given the following reactions and subsequent $H$ values: $(4$ marks$)$

$C{O}_{2(g)}{C}_{(s)}+O_{2(g)}{H}_1=393\mathrm{.}5kJ$

$2H_2{O}_{(g)}2H_{2(g)}+O_{2(g)}$

$8C_{(s)}+9H_{2(g)}{C}_8{H}_{18(g)}$

$H_2=483\mathrm{.}6kJ$

$H_3=-250\mathrm{.}1kJ$