$$\eqalign{
& b)\,\,{3.5^{2x - 1}} - {2.5^{x - 1}} = 0,2 \cr
& \Leftrightarrow {3.5.5^{2x - 2}} - {2.5^{x - 1}} = 0,2 \cr
& Dat\,\,t = {5^{x - 1}}\,\,\left( {t > 0} \right) \Rightarrow {t^2} = {5^{2x - 2}} \cr
& PT:\,\,15{t^2} - 2t - 0,2 = 0 \cr
& \Leftrightarrow \left[ \matrix{
t = {1 \over 5}\,\,\left( {tm} \right) \hfill \cr
t = - {1 \over {15}}\,\,\left( {ktm} \right) \hfill \cr} \right. \cr
& \Leftrightarrow {5^{x - 1}} = {1 \over 5} = {5^{ - 1}} \Leftrightarrow x - 1 = - 1 \Leftrightarrow x = 0 \cr} $$
