If $a=2$ and $b=3$, find the value of $ab-a^{2}$.

Given: $a=2$ and $b=3$.

To do: To find the value of $ab-a^{2}$.

Solution: 
 

$ab-a^{2}$

$=a( b-a)$

$=2( 3-2)$

$=2( 1)$

$=2$

Thus, $ab-a^2=2$.