kill(all)$

van=facebook.com/LearningCalculus$

log5(x):=log(x)/log(5)$

f(a):= 5^(-a) - 25;

a:rhs(first(solve(f(a),a)));

a^5;

f(x)=y;

x = log5(1/(y+25));

solve(rhs(%)=0,y);