Consider the following reaction:
 ${\mathrm{N}}_2{\mathrm{O}}_4\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{NO}}_2\left(\mathrm{g}\right);{\mathrm{\Delta H}}^0=+58\mathrm{kJ}$
For each of the following cases (a,b), the direction in which the equilibrium shifts is.
a) Temperature is decreased
b) Pressure is increased by adding ${\mathrm{N}}_2$  at constant  p.
 

Increase in temperature favour forward reaction i.e. endothermic, Decreaese in temperature favour backward reaction 
Adding increases  $\left({\mathrm{N}}_2\right)$ not participating in given reaction shifts the equilibrium towards more number of moles side at constant pressure.