Byron S. answered 10/14/14
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Math and Science Tutor with an Engineering Background
You can find the yield per acre for the orchard by multiplying the apple production per tree times the tree density per acre. If you add x to the density, you must subtract 2x from the yield.
Y = (36-2x)*(22+x)
You'll want to multiply this together and then find the vertex of this parabola. The x2 term is negative, so it opens downward, which makes the vertex the maximum.
If you've done derivatives, take the first derivative, and set it equal to 0 to find the value of x that maximizes Y. You should then plant (22+x) trees/acre.
I hope this helps, please comment if you have further questions!