From what you are told you will find that the yeild per tree:

y = 31 - (t-51), for t<=71

where t is the number of trees per acre. therefor the total yield per arcre:

Y= t*y = 31*t - t*(t-51) = 72*t - t^2

This takes a maximum at the point where dY/dt=0, or an end point of the interval.

dY/dt = 72 - 2t,

so dY/dt = 0, gives us t=36 with a yeild of 1296 pounds per acre, we also

check the end point t=71, to get yeilds per acre of : 71.

So the maximum yeild is 1296 pounds per arce at a tree density of 36 trees pre acre.

RonL