So as the path adds an extra 2 feet to each side I would start this by adding two feet to each part (so 25+2 x 38+2) which leaves you with 27ft by 40ft. Multiply to find this total area, which results in 1080. Now, you want to remove the actual area of the garden itself as this is not part of the path. So 25x38=950. Subtract 1080 by 950. This gives you just the area of the gardens path which would be 
Sn = (a1) x (1 - r^n) / (1 - r)
Substituting the known values:
S5 = (6) x (1 - (1/3)^5) / (1 - 1/3) = 242/27 = 8 24/27 = 8 8/9
ANSWER
-2 shift the graph of the basic function down by 2 units.
EXPLANATION
The given cosine function is:

This equation can be rewritten as:

We compare this to

The effect d has on the graph is that, it shifts the graph up by d units.
If d is negative the graph shifts down by d units.
Since d=-2, the graph will shift down by 2 units.