Vf-vi/t solve for vf

For vi:

multiply each side by T A t = (vf - vi)

minus VF from each side A t - vf = - vi

Take - 1 and multiply by each side vf - A t = vi

For vf:

again multiply each side by T : A t = (vf - vi)

then take VI and add to each side A t + vi = vf