Consider the following processes : 

$\begin{array}{lc} & \mathrm{\Delta H}(\mathrm{kJ}/\mathrm{mol})\\ 1/2\mathrm{A}\to \mathrm{B}& +150\\ 3\mathrm{B}\to 2\mathrm{C}+\mathrm{D}& -125\\ \mathrm{E}+\mathrm{A}\to 2\mathrm{D}& +350\end{array}$

For $\mathrm{B}+\mathrm{D}\to \mathrm{E}+2\mathrm{C},\mathrm{\Delta H}$ will be:

Given $\mathrm{\Delta H}$

$\begin{array}{l}\frac{1}{2}\mathrm{A}⟶\mathrm{B}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}+150 .....\left(1\right)\\ 3\mathrm{B}⟶2\mathrm{C}+\mathrm{D}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-125 ......\left(2\right)\\ \mathrm{E}+\mathrm{A}⟶2\mathrm{D}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}+350 ......\left(3\right)\end{array}$

To calculate $∆$H operate

$\begin{array}{l}2\times \text{ eq. (1) }+\text{ eq. }\left(2\right)-\text{ eq. (3) }\\ \mathrm{\Delta H}=300-125-350=-175\end{array}$