$x,yR(x),$ where $R(x)$ represents revenue $(in$ thousands $of$ dollars$)$ and $x$ represents the amount spent $on$ advertising $(in$ thousands $of$ dollars$).$

$R(x)=11,000-x^3+36x^2+800x,0x20$

The point $of$ diminishing returns $is$

$.$

$(Typean$ ordered pair. $Do$ not use commas $in$ the individual coordinates.$)$