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**electricity problem :(**

so heres the problem:

through what potential difference would an electron need to be accelerated for it to achieve a speed of 42.0% of the speed of light, starting from rest? The speed of light is c = 3.00e8 m/s

so i thought the equation i would use would be Vf-Vi=deltaPE / q

so i thougth the change in potential energy would be equal to the opposite of change in kinetic energy, so delta PE would = .5mv^2, and v is 3e8 X .42, so:

.5 X 9.11e-31 X (1.26e8)^2 / 1.6e-19 = 4520V, but this is wrong, so yea, no clue. any help would be much appreciated, thanks