DeLorean?

A car travels north with speed $3 \times 10^8 \text{ m/s}$ which is speed of light and covers exactly $300 \text{ m}$ in north direction, the driver suddenly noticed that he is slow and he doubles his initial speed at that point but the car changes its direction and moves to south and covers displacement of $600 \text{ m}$ . It also moves $400 \text{ m}$ further down.

What is the resultant displacement?

Assume the resultant force is kept $900000 \text{ N}$ through out the system .