Question 1035557
Too open.  Choose any value you want between 0 and 12 as a divisor.  Use some values that are practical.  How about 2?  {{{2*2+2*d=12}}} and d is the other dimension for the rectangular region to be fenced; but only an example.


To continue with this example,  2 units by d units, the area would be {{{2d}}} square units.


Grade 3 level of basic mathematics question? 
The question NEEDS to include what shape is this yard to be fenced.  I assumed a rectangle.  An example was chose as one direction being 2, so this would be for two opposites sides of the assumed rectangle.  These account for length of 4 length units.  You know the distance around the yard must be 12 units, leaving 8 more units to be accounted.  The other two opposite sides would together be this 8 units, so because there are two of them, the 8 is cut into two equal parts, so 4 units.   Area would be  {{{2*4=8}}} square units.



The number of combinations you can find can be many.  I chose 2 because it is a WHOLE number, so this should be understandable for grade 3 person, but grade 3 is probably also the time when a student starts to learn about fractions.  You could choose a mixed number value greater than 0 but less than 12, and analyze the combination starting with the chosen value.