Suppose that:

$$ f $($x$)=($x$3)$ with constants B and x$1>=1.$

$$ g$($x$)=$ O$($x$)$ using constants C and x$2>=1.$

$$ g$($x$)$ is always positive.

For appropriate constants D and x$3$ we can prove that:

$5$f $($x$)/$g$($x$)+$ f $($x$)/2$x $=($x$2)$

Not knowing anything else, what is the minimum value of x$3$ and the maximum value of D$?$