Answer at very bottom of picture above
if explanation need comment
Let y = f(x)
y = ln(3x)
Exchange place with x and y.
x = ln(3y)
Solve for y.
y = (e^x)/3
Replace y with the inverse notation.
f^(-1) x = (e^x)/3
Done.
<span>Kevin is 16
Timothy is 10
------------
check:
4 yrs ago Kevin was 12
4 yrs ago Timothy was 6
Kevin was twice his age
OK</span>