Question

An apple orchard has an average yield of 32 bushels of apples per tree if tree density is 24 trees per acre. For each unit increase in tree density, the yield decreases by 2 bushels per tree. How many trees per acre should be planted to maximize the yield? = trees
starting point = 24 +t trees where t = 0
b = bushels
stating point = 32 -2t where t = 0

yield = (24+t)(32-2t)

y = 768 -16t - 2t^2

y' = -16 -4t

0 = -16 -4t

t = -4

plant 20 trees how did the answer go from -4 to 20?