You're correct, the answer is C.
Given any function of the form

, then the derivative of y with respect to x (

) is written as:

In which

is any constant, this is called the power rule for differentiation.
For this example we have

, first lets get rid of the quotient and write the expression in the form

:

Now we can directly apply the rule stated at the beginning (in which

):

Note that whenever we differentiate a function, we simply "ignore" the constants (we take them out of the derivative).
X = 3 and x = 4 will make the polynomial = 0
answer
C. 3 and 4
875 x 12 = 10500ft squared
950 x 6 = 5700ft squared
10500 + 5700 = 16200
the total floor area is 16200 squared feet