Remark
Let's assume that each square consists of 4 matchsticks. If this is incorrect, leave a note. We can't really see how many match sticks make up a square, so it makes 6c hard.
Solution
First write the number of squares in the nth windmill.
d = 4 (that's derived from 13 - 9 which are the second and third term.
n = n
a = 5
an = a + (n - 1)*d
an = 5 + (n - 1)*4
Now each square is made up of 4 matchsticks.
So you need to multiply "an" by 4 to get the number of matchsticks.
an_match = 4*[5 + (n -1)*4] Remove the inner brackets
an_match = 4*[5 + 4n - 4]
an_match = 4*[4n - 1] Remove the square brackets.
an_match = 16n - 4 <<<< Answer