There are three persons $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ in a room. If a person $\mathrm{D}$ joins the room, the average weight of the persons in the room reduces by $x \mathrm{~kg}$ . Instead of $\mathrm{D}$ , if person $\mathrm{E}$ joins the room, the average weight of the persons in the room increases by $2 x \mathrm{~kg}$ . If the weight of $\mathrm{E}$ is $12 \mathrm{~kg}$ more than that of $\mathrm{D}$ , then the value of $x$ is