kill(all)$

van=facebook.com/LearningCalculus$

f(x,y):=2^x-3^y=7;

solve(f(x,y),y);

f(3,0);
f(4,2);

      After Wolfram et. al.

y(x,n):=fullratsimp(%i*(2*%pi*n-%i*log(2^x-7))/log(3));

y(x,0);

y(x,1);

y(x,2);