Final answer:
The farmer calculates the length of side D to complete the perimeter of his four-sided plot of land. The perimeter is the total length of all sides, and side D can be determined by rearranging the equation y + A + B + C + D = 0, which results in D = -y - A - B - C.
Step-by-step explanation:
The question relates to the concept of finding the perimeter of a polygon in mathematics. The perimeter of a polygon is the total length of all its sides. When calculating the perimeter, one would sum the lengths of all the individual sides. In the student's problem, there is a four-sided plot of land measured by sides A, B, C, and D. If sides A, B, and C are known, then side D can be calculated such that the sum of all sides A + B + C + D equals the perimeter of the plot. Here we have the equation y + A + B + C + D = 0, we can assume that 'y' represents another expression related to side D (for example, y could be the negative value representing the opposite side of D). To find D, we would rearrange this equation to solve for D. If you subtract y, A, B, and C from both sides of the equation, you will get D = -y - A - B - C, which represents the length of side D. Therefore, the farmer would use this result to determine the length and orientation of side D for the perimeter of his plot.