There are many ways to answer this but, I'll try to explain it.
You can see that you are computing a limit when some n approaches negative PI. You get to substitute negative PI instead of n and compute cos(-pi) which is -1.
Then compute f(x) - (-1) = 3 - (-1) = 4 which is a result.
The way you can imagine cos(-pi) or cos(-90deg) is by dragging point in a unit circle beneath x-axis and to the bottom y-axis (-90 angle), the y coordinate tells you the cosine value at (-90) and because you have dragged the point on the unit down to y-axis the value is -1.
Hope this helps.
