My guess would be
the missing number is $3$
If we denote the four numbers by $a,b,c,d$, in clockwise order, starting with the lower left position (so that in the left picture $a=2$, $b=8$, $c=14$ and $d=9$), then these number satisfy the relation
$d*a-(b/a) = c~~~~$ Indeed, $9*2-(8/2)=14$ in the left picture.
In the right picture, the unknown $x=?$ then should satisfy $x*3-(15/3)=4$, which yields the answer $x=3$.