Answer
$-36.03\ kJ/mol$
Work Step by Step
From the definition of heat capacity:
$(415\ g\times 4.184\ J/g.K+155\ J/K)\times(20.72-18.90)\ K=3442.3\ J$
With a negative sign since the temperature raises.
Number of moles of ammonium nitrate:
$7.647\ g\div80.04\ g/mol=0.096\ mol$
Hence:
$-3442.3\ J\div 0.096\ mol=-36.03\ kJ/mol$
