You can find the inverse by swapping x and y values then solve for y.
In your problem y = 6^.5x so swap and get:
x = 6^.5y
In order to solve for y, you will need to take the Ln of both sides and use the law of logs to "free up" y:
Ln(x) = Ln(6^.5y) = (.5y) * Ln(6)
Now just solve for y. Hope that helps