3^x - 3^(x+1) = 5^x - 5^(x+1);
[3^x] - [(3^x) * (3^1)] = [5^x] - [(5^x) * (5^1)];
(3^x) * [1 - 3^1] = (5^x) * [1 - 5^1];
(3^x) * (- 2) = (5^x) * (- 4);
(3^x)/ (5^x) = (-4) / (- 2);
(3/5)^x = + 2;
ln[(3/5)^x] = ln(2)
x * ln(3/5) = ln(2)
x = ln(2) / ln(3/5).
Ciao @alby