Explanation

(a)
The heat of formation of $CO$ is calculated by using Hess’s law. According to it, the total heat changes occurring during a chemical reaction are independent of path.
$2CO(g) + O_{2}(g) \rightarrow 2{CO}_{2}(g);$ $\mathrm{\Delta}H = - 135.2$ kcal
(I)$CO_{2}(g) \rightarrow CO(g) + \frac{1}{2}O_{2}(g)$; $\mathrm{\Delta}H = \frac{135.2}{2}$ kcal
(II)$\ C(s) + O_{2}(g) \rightarrow CO_{2}(g);$ $\mathrm{\Delta}H = - 94$ kcal
Required equation
$C(s) + \frac{1}{2}O_{2}(g) \rightarrow CO(g);$ $\mathrm{\Delta}H = ?$
Add Eqs. (I) and (II)
$C(s) + \frac{1}{2}O_{2}(g) \rightarrow CO(g)$; $\mathrm{\Delta}H = - 26.4$ kcal