Sorry guys, okay well this is what I tried myself:
y = 5x/(x + 3)
Next, switch the x and y variables, and re-write the function as:
x = 5y/(y + 3)
Now, solve this equation for y:
x(y + 3) = 5y
xy + 3x = 5y
3x = 5y - xy
3x = y(5 - x)
y = 3x / (5 - x)
Solution: f^-1(x) = 3x / (5 - x)
Thanks for the advice, I did work it out right didn't I?