Answer:
y = 3x + 4
Step-by-step explanation:
According to the given question, the expression to represent all the books in Ms. Canton's bookcase is shown below:-
y indicates the total amount of books
x indicates the equal cost of books on 3 bookshelves
while
+4 indicates 4 other books on the fourth bookshelf
So, the expression will be
y = 3x + 4
Therefore the correct answer is y = 3x + 4
Given

To obtain the minimum value of y, we first take the derivative of y
The derivative of y is:

Equating

gives the minimum value we require.
Doing that, we have:

So that

Therefore, the minimum value is x = 3