Answer: 800 kJ $mol^{-1}$

$X-X + Y-Y \rightarrow 2 X-Y$ ; $\Delta H = -200 \times 2 $ kJ

Let, B.E. of X-X =a

B.E. of X-Y = a

and B.E. of Y-Y = a/2

$\Delta H = -400$ kJ

$\Delta H = B.E. (X-X) + B.E. (Y-Y) - 2 \times B.E. (X-Y)$

or, -400 kJ = a + 0.5 a - 2a

or, 0.5 a = 400 kJ

or, a = 800 kJ

B.E. of $X_2$ = a = 800 kJ $mol^{-1}$