Missing Numbers - 9

If we assume that the numbers on vertical line are $a,b$ and the numbers on horizontal line are $c,d$ and the number in the circle is $e$ then the relation you will find is : $ab - cd = e$ . Lets check it :

$\begin{aligned} (8 \times 5 - (4 \times 3) & = 28 \\ (12 \times 7) - (8 \times 9) & = 12 \\ (11 \times 3) - (6 \times x) & = 21 \\ 6x & = 12 \\ \implies & \boxed{x = 2} \\ \end{aligned}$