First we need to find what the revenue function is: given that the price function is
p(x)=22-x and x is the quantity of violins demanded
Then the revenue from people buying the violins will be
R(x)=xp(x)=22x-x^2.
Now, the marginal revenue is given by R'(x)
R'(x)=22-2x
a)
R'(5)=22-2(5)=12
So, when 5 violins are demanded, the companies revenue is increasing by $12,000 per violin demanded.
b)
R'(15)=22-2(15)=-8
So, when 15 violins are demanded, the companies revenue is decreasing by $8,000 per violin demanded.
c)
R'(13)=22-2(13)=-4
So, when 13 violins are demanded, the companies revenue is decreasing by $4,000 per violin demanded.