To find C you note that it is a turning point.

Thus,

Since,

The derivative is, (product rule)

Use the chain rule now,

Factor,

Simplfy,

You can factor again,

Set equal to zero,

Divide by 6,

Each factor equal to zero,

One of these points is C and one is D, but which one is it?

Note C is to the left of D thus C<D.

Thus,