The perimeter of a rectangular garden is 860 ft. the two short sides of the garden are each 30 ft long. you are asked to find the length of the other sides. which equation models this situation?

2(30)+2x=860 

This is because the 4 sides of the rectangle add up to the perimeter. 
The unknown side is x, and there are two of those, and there are also two sides of 30. 
They all add up to the perimeter. 

So: 
2(30) + 2x = 860