Question 1036983
Maximize the function (90+x)(1500-9x)
This is -9x^2+690x+135000
Set it equal to 0, and the vertex, or the x value of the vertex, which is wha is needed, is -b/2a.  This is -690/-18=38.3333 or 38 additional trees.

{{{graph(300,200,-100,100,-100,200000,-9x^2+690x+135000)}}}