Answer
$36$ trees
$5256\ lb$ per acre.
Work Step by Step
Step 1. Assuming the number of fruit trees is $x$ with $x\geq35$, the yield per tree then is given by
$150-4(x-35)=290-4x$
Step 2. The annual yield is given by
$Y=x(290-4x)=-4x^2+290x$
Step 3. The maximum yield happens at
$x=\frac{290}{8}=\frac{145}{4}\approx36$
Step 4. The maximum yield is
$Y=-4(\frac{145}{4})^2+290(\frac{145}{4})=5256.25\approx5256\ lb$ per acre.
