Potentially, what's the point?

Consider a 1 nC point charge as shown below:

• Find the electric potential at points a and b.
• Find the electric potential difference between points a and b.
• Find the electric potential energy of a proton placed at points a and b.
• The speed of a proton at point b that at point a was moving to the right with a speed of 4e5 m/s.
• The speed of a proton at point a that at point b was moving to the left with a speed of 4e5 m/s.

Now consider a proton moving to the left along the trajectory shown below:

The proton's speed at point c is 4e5 m/s. What is the proton's speed at point d?