Question

d=1/2 (Vf+Vi)t solve for t

Answer 1

You said that                              D = 1/2 (Vf + Vi) t

Multiply each side by  2 :           2D = (Vf + Vi) t

Divide each side by  (Vf + Vi) :  2D / (Vf + Vi) = t


This is simple algebra.  You really should try to do
the remaining ones on your own.