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

You said that                              D = 1/2 (Vf + Vi) tMultiply each side by  2 :           2D = (Vf + Vi) tDivide each side by  (Vf + Vi) :  2D / (Vf + Vi) = tThis is simple algebra.  You really should try to do the remaining ones on your own.