Write an equation in standard form for: the perimeter of a rectangle that is x units wide and y units long is 24 centimeters

Hello!
When finding the perimeter of a rectangle, you have to consider the properties of a rectangle. A rectangle has two pairs of equal sides where one is the width, while the other one is the length. 
Now looking back at your question, it says "... a rectangle that is x units wide" ⇒ you let the width = x ; this is the same with "y units long" ⇒ length = y. Perimeter can just be : P = 24.  
Therefore,
The equation would be:
x + x + y + y = P. 
2x + 2y = P.
(Sub in P = 24)
∴ 2x + 2y = 24. (This should be your answer.)
:) Good luck (Message me if you have any problem)