The length of a rectangular field is 12 yards less than the field's width. if the sum of the length and width is 96 yards, what is the length of the field in yards

This answer is vague in whether they want you to add just the length and the width or if they are asking that the perimeter is 96 yards. I will show both. 
The length is width - 12. Lets call width x.
Length = x - 12
Width = x
So if it is just the sum of the length and width, it would be x + x -12 = 96
2 x - 12 = 96
2 x = 108
x = 54
Length would be 54 - 12 = 42 yards.
If it was perimeter, the values would be the same except 2(x) + 2(x-12) = 96
2x + 2x -24 = 96
4x = 120
x = 30
Length = 30 - 12 = 18 yards
I hope this helps :-)