Answer: The length of side DE is 9 units.
Step-by-step explanation: Given that the co-ordinates of the end-points of DE are D(0, 3) and E(0, 12).
We are to find the length RT of the polygon.
We know that
the length of a line segment with endpoints P(a, b) and Q(c, d) is equal to the distance between the points P and Q.
By distance formula, the distance between P(a, b) and Q(c, d) is

So, the distance between D(0, 3) and E(0, 12) is given by

Thus, the required length of DE is 9 units.