9^x+2*3^x=15 помогите!!!

спасибо огромное !!!

9^x + 2•3^x = 15 3^x = a ( a > 0 ) a^2 + 2a - 15 = 0 D = 4 + 60 = 64 = 8^2 a1 = ( - 2 + 8 ) : 2 = 3 a2 = ( - 2 - 8 ) : 2 = - 5 ( < 0 ) 3^x = 3 X = 1