${\mathrm{A}}_2\left(\mathrm{g}\right)+{\mathrm{B}}_2\left(\mathrm{g}\right)\stackrel{ }{\rightleftharpoons }3{\mathrm{X}}_2\left(\mathrm{g}\right),\mathrm{\Delta H}=+\mathrm{XkJ}$

According to Lechatelier's principle, favourable conditions for the decomposition  of ${\mathrm{X}}_2$ are

Forward  reaction is endothermic $\therefore$High T favours forward reaction

  Since product formation involves increase in volume (no. of moles) , low pressure favours product formation.

But decomposition of ${\mathrm{X}}_2$ is the backward reaction,

It is favoured at  Low T and  High P