Why Does Mastering Physics Show an Error in My Potential Energy Calculation?

[fcman]

Mastering physics keeps saying I'm off by an additive constant.

A proton's speed as it passes point A is v. It follows the trajectory shown in the figure.

This is my input: sqrt(80/(1.67*10^-27))+v

 

[fcman]

Nothing? This thing is due in a couple hours