RIOPLATENSE 1993 Problem 4

$x$ and $y$ are real numbers such that $6 -x$ , $3 + y^2$ , $11 + x$ , $14 - y^2$ are greater than zero. Find the maximum of the function $$f(x,y) = \sqrt{(6 -x)(3 + y^2)} + \sqrt{(11 + x)(14 - y^2)}.$$