Do you know that exponentials and logarithms are inverses of each other?
y = ax [itex]\Longrightarrow[/itex] x = logay
Then, after substituting the x, [tex]y = a^{\log_a y}[/tex]
So what is [tex]5^{\log_5 x}[/tex] ?
Perhaps you should review http://en.wikipedia.org/wiki/Logarithm" .