I don't think you need to your calculus for this problem. Just understanding how the parameters A,B, and C affect the shape of the graph of the cosine function should suffice. C is the amount the graph is shifted up and or down while A is the amplitude of the graph (the amount the function goes up from C and down from C).

So draw a standard cosine graph. Shifting the graph up by 1 amounts to adding 1 to the cosine function, in other words setting C = 1.

In this shifted cosine function the graph oscillates between 2 and 0 instead of 1 and -1.

Note that the value of C is the midpoint between 2 and 0 the max and the min, this is not an accident.

The value of B changes how quickly the function oscillates (a correct choice for this value will ensure that the correct number of critical points appear in the interval (0,4))