Answer
$\Delta H^o_f : CaC_2(s) = -60.7kJ$
Work Step by Step
$CaC_2(s) + 2H_2O(l) --> Ca(OH)_2(s) + C_2H_2(g)$
$\Delta H^o = -127.2kJ$
$\Delta H^o_f : H_2O(l) = -285.83 kJ/mol$
$\Delta H^o_f : Ca(OH)_2(s) = -986.2 kJ/mol$
$\Delta H^o_f : C_2H_2(g) = 226.7 kJ/mol$
$\Delta H^o_f : CaC_2(s) = x$
$-127.2 = [-986.2 +(226.7)] - [x + 2*(-285.83)]$
$-127.2 = -759.5 - [x - 571.66]$
$-127.2 +759.5= -x + 571.66 $
$632.3 = -x + 571.66$
$60.7 = -x$
$x = -60.7 kJ$