An astronomical survey team recorded the distance from Earth and the corresponding recessional velocity of several distant galaxies in a newly observed sector. Figure 1 shows their scatter plot along with a line of best fit.
| Distance from Earth (Mpc) | Recessional velocity (km/s) |
|---|---|
| 2.0 | 150 |
| 5.0 | 310 |
| 8.0 | 560 |
| 11.0 | 730 |
| 14.0 | 970 |
| 17.0 | 1180 |
| 20.0 | 1340 |
Calculate the gradient of the line of best fit shown in Figure 1.
State the unit.