When a triangle is translated (moved) in the coordinate plane, the lengths of its sides do not change. Therefore, even though triangle XYZ is moved down 4 units and to the left 8 units, the length of side XY remains the same.
Since the length of side XY is 10 units, the length of side X′Y′ (the corresponding side after the translation) will also be 10 units.
So, the length of side X′Y′ is 10 units.