A=(vf-vi)/t solve for t
solve for vi
solve for vf

A = (vf - vi) / t    <== call this the "given equation"



For t :

Multiply each side of the given equation by 't' :    A t = (vf - vi)

Divide each side by 'A' :                                        t = (vf - vi) / A



For vi :

Multiply each side of the given equation by 't' :    A t = (vf - vi)

Subtract ' vf ' from each side:                             A t - vf = -vi

Multiply each side by  -1 :                                  vf - A t = vi


For vf :

Multiply each side of the given equation by 't' :    A t = (vf - vi)

Add ' vi ' to each side:                                      A t + vi = vf