Answer
$50$ trees per acre.
Work Step by Step
We can rewrite the function in our usual form:
$A(n)=-9n^2+900n$
In this function, $a=-9$, $b=900$, $c=0$.
The maximum number of apples per acre occurs at:
$n=\frac{-b}{2a}=\frac{-900}{-18}=50$
(calculating maximum yield is unnecessary).
In conclusion, to reach the maximum number of apples per acre, $50$ trees should be planted per acre.
