A group of astronomers measured the distances to several distant galaxies and their recessional velocities. Figure 1 shows the scatter plot with a line of best fit.

| Distance from Earth (Mpc) | Recessional velocity (km/s) |
|---|---|
| 2.0 | 120 |
| 4.0 | 320 |
| 6.0 | 410 |
| 8.0 | 620 |
| 10.0 | 710 |
| 12.0 | 920 |
| 14.0 | 1010 |
Calculate the gradient of the line of best fit shown in Figure 1.
State the unit.