It's a function of function f(g(x))
f(x) = (x-3)/x and g(x) =5x-4
Just plug the value (5x-4) in x of f(x)
f(x) = [(5x-4) -3] / (5x-4) ==== f(x) = (5x-7)/(5x-4)
The domain are all the value of x different from 4/5 (or the number that render the denominator nil.
In short 5x-4 ≠ 0 or x≠ 4/5
Domain ={x/x ≠5/4}