With this information how do I find the theoretical value and % error? Slope: 2.8 y intercept: (0,-18)

Data: Time (s) VBlue (Unit] VRed (Unit) 3s -9.6 m/s -0.5 m/s 6s -1.2 m/s -5 m/s 9s 7.2 m/s -9.5 m/s 12s 15.6 m/s -14 m/s 15s 24 m/s -18.5 m/s 18s 32.4 m/s -23 m/s *Write in the appropriate units for each column