A) write a relation for perimeter of a square with side length n centimetres

The perimeter of a square is equal to the side length times 4.

The equation is P = 4S, with P being the perimeter, and S being a side. 

Also, to get the length of a side from the perimeter, just divide the perimeter by 4.

To have it in centimeters, just add cm to the end of your answer!