Answer: B(3)
Explanation: 12+9=21 so since 4+3 only equals 7, you plug in x as 3... since 7x3 is 21 that helps make the statement true bc both equations are equal.
Marginal Revenue is defined as the revenue generated with respect to increase in sale of another extra unit of product.
So we will calculate Marginal Revenue equation by finding the change in Revenue with respect to change in Quantity.
Given the Total Revenue Function as:-

Differentiating Total Revenue function (Tr) with respect to quantity (q):-
So Marginal Revenue equation will be, 
Difference = 3,5,7,9,11...
2nd difference = 2
difference = (2n+1)
gradient = 2n
integral of 2n = n^2.
n^2 = 1,4,9...
n^2 - 1