For the reversible reaction,
$N_2\left(g\right)+3H_2\left(g\right)\iff 2N{H}_3\left(g\right)$ + heat
The equilibrium shifts in forward direction

Any change in the concentration, pressure and temperature of the reaction results in change in the direction of equilibrium This change in the direction of equilibrium is governed by le-chateller's principle. According to this equilibrium shifts in the opposite direction to undo the change.
$N_2\left(g\right)+3H_2\left(g\right)\iff 2N{H}_3\left(g\right)+Heat$
(a) Increasing the concentration of $N{H}_3\left(g\right)$: On increasing the concentration of $N{H}_3\left(g\right)$ , the equilibrium shifts in the backward direction where concentration of $N{H}_3\left(g\right)$ decreases.
(b) Decreasing the pressure: Since $p\propto n$ (numgber of moles) , therefore equilibrium shifts in the backward direction where number of moles are increasing.
(c) Decreasing the conentraion of $N_2\left(g\right)$ and $H_2\left(g\right)$ : Equilibrium shifts in the backward direction when concentration of $N_2\left(g\right)\text{and}{H}_2\left(g\right)$ decreases.
(d) Increasing pressure and decreasing temperature : On increasing pressure, equilibrium shits int he forward direction where number of moles decreases. it is an eaxample of exothermic reaction therefore decreasing temperature favours the forward directions.