Answer
$\Delta H $ = -630 kJ
Work Step by Step
C6H6 --> 3 C2H2 $\Delta H = + 630 kJ $
If we want to find the enthalpy change for the reverse reaction, we would need to multiply the enthalpy by negative 1 since we are reversing the direction. This means
$\Delta H $ = -1 * 630kJ = -630 kJ