We want the reaction $S + O_2 \rightarrow SO_2$. From (1), $S + 3/2 O_2 \rightarrow SO_3$ with $\Delta H = -2x$. Reverse (2): $SO_3 \rightarrow SO_2 + 1/2 O_2$ with $\Delta H = +y$. Adding these two reactions gives $S + O_2 \rightarrow SO_2$ with $\Delta H = y - 2x$. The heat of formation is $y-2x$ kcal. [cite: 1, 1569]