If f (x) = x - 3 / x and g (x) = 5x - 4, what is the domain of (fog) (x)

You must plug the g (x) function into every x in f (x) ...

((5x) - 3) / (5x) = (fog) (x)

Since the only value that would be undefined is x = 0 which makes the denominator zero, the domain is all reals except x = 0.

Or

(-∞,0) U (0,∞)