Enthalpy calculation
$$\ce{X + 2Y -&gt; 2Z}$$


\begin{align}
\ce{W + X &amp;-&gt; 2Y} &amp;\qquad &amp;\Delta_\mathrm{r}H_1 = \pu{-200 kcal} \tag{1}\\  
\ce{2W + 3X &amp;-&gt; 2Z + 2Y} &amp;\qquad &amp; \Delta_\mathrm{r}H_2 = \pu{-150 kcal} \tag{2}
\end{align}

$(-1) \times 2$ + $(2)$ gives: 

$\ce{4Y +2W + 3X-&gt;2W + 2X + 2Z + 2Y} \equiv \ce{2Y +X -&gt; 3Z}$

Using Hess's Law,
the enthalpy change for the required reaction is given by  $-2\times \Delta_rH_1+\Delta_rH_2= 400-150 = \pu{250 kcal}$