Let's take this one a step at a time.

We'll start by simply drawing the graph of y = cos 2x.

a = 1; period = 2π/2 = π.

Now, let's shift the curve (we are now considering the "minus π" part of the question.

displacement = -c/b = -(-π)/2 = π/2

So we have to shift every point on the curve to the right (because phase shift is a positive number) by π/2. This give us y = cos(2x - π), the blue dotted curve.

Now we need to consider the minus out the front of the expression y = -cos(2x - π). The minus will just give us a mirror image in the x-axis, since every positive value becomes negative and every negative value becomes positive. In other words, the minus turns it upside down.

The answer for the graph of y = -cos(2x - π) is the red curve:

But wait, this is what we started with! It looks the same as y = cos 2x.

This example shows an interesting thing about phase shift and periodic functions. If you shift far enough, you can easily obtain equivalent sine or cosine expressions.

You will get a better understanding of why this works in Sums and Differences of Angles, which we meet later.