Since you are already given the formula for your arithmetic sequence you only gotta plug stuff in, and you have the following
The Perimeter of a Rectangle.
It's the sum of all its sides.
L + W + L + W or 2(L + W)
The square root of 20? You'd use Pythagorean Theorem.
C. is the correct answer, because both numbers are increasing.
Therefore, C. represents a linear increasing function.