Using the law of sines,
sin80°/3.1 = sin∠QSR/2.4
∠QSR = sin^-1(2.4sin80°/3.1)
≈ 49.679°
∠SQR = 180° - 80° - 49.679° (angles in a triangle)
= 50.321°
sin50.321°/RS = sin80°/3.1
RS = (3.1sin50.321°/sin80°)
= 2.4 units (nearest tenth)
The length of line RS is 2.4 units.