The length of a side of a triangle remains unchanged when the triangle is translated. A translation shifts the entire triangle without altering its size or shape.
In this case, triangle XYZ is translated down 4 units and to the left 8 units. Since these movements are translations, the length of side XY, which is 10 units, will remain the same after the translation.
Thus, the length of side X'Y (after translation) is still 10 units.