Factorize each of the following expressions:

$a b-b y-a y+y^{2}$

$=(a b-a y)+\left(y^{2}-b y\right)$            [Grouping the expressions]

$=a(b-y)+y(y-b)$

$=a(b-y)-y(b-y)$       $[\because(\mathrm{y}-\mathrm{b})=-(\mathrm{b}-\mathrm{y})]$

$=(a-y)(b-y)$            $[$ Taking $(b-y)$ as the common factor $]$