After a while, i finaly got to this equation, and solved it.
It was quite easy.
x*3^{log_{x}5}=15 /multiply this with log of base 3
log_{3}x + log_{x}5 = log_{3}5 + 1
when you convert
log_{x}5
to base 3, and and multiply the whole equation with
log_{3}x
and sort it out, factorize, you...