The work W done by the electric field in moving the proton is equal to the difference in electric potential energy of the proton between its initial location and its final location, therefore:
[tex]W= qV_i - qV_f[/tex]
where q is the charge of the proton, [tex]q=1 e = 1.6\cdot 10^{-19}C[/tex], with [tex]e[/tex] being the elementary charge, and [tex]V_i = +125 V[/tex] and [tex]V_f = -55 V[/tex] are the initial and final voltage.
Substituting, we get (in electronvolts):
[tex]W=e(125 V-(-55 V))=180 eV[/tex]
and in Joule:
[tex]W=(1.6 \cdot 10^{-19})(125 V-(-55V))=2.88 \cdot 10^{-17}J[/tex]
