What is the Potential difference?

[mrlucky0]

Homework Statement

As a positron moves through a region of space, its speed decreases from 9.20*10^6 m/s to 2.20*10^6 m/s. The electric force is the only force acting on the positron.

Homework Equations

KE1 + U1 = KE2 + U2
Where KE is kinetic energy and U is potential energy.

The Attempt at a Solution

Using the above equation I have:

Potential Difference = $$\\text{\Delta v} = \frac{1/2m\left(v_f^2-v_i^2\right)}{q}$$

Where m is the mass of the positron, 9.1E-31 kg, and q is the charge of the positron, 1.6E-19 C. The v's are the final and initial speeds. The potential difference comes out to be: 77 V when I solve it.
I am not getting the correct answer. Am I proceeding about this the right way?

 

[learningphysics]

It should be vi^2 - vf^2 in the numerator... but other than that everything looks right. check your calculations... I'm getting a different number.