The width of a rectangular garden is 99 feet, and its length is 1212 feet. Three of these expressions equal the perimeter of the garden, in feet. Which expression does NOT?

You have not provided the choices, therefore, I cannot provide you with a specific solution. However, I will explain this problem for you to understand an apply on the choices you have.

Now, the perimeter of the rectangle is the summation of lengths of its four sides.
We have:
length = 12 ft
width = 9 ft

Therefore:
perimeter = 12 + 9 + 12 + 9 = 42 ft
Or: perimeter = 2(12+9) = 2*21 = 42 ft
Or: perimeter = 2*12 + 2*9 = 24 + 18 = 42 ft

Any expression that does not lead to the above result would be wrong

Hope this helps :)