f(x)^-1=(Y-4)/3 Mr F says: Wrong. The inverse is a function of x, not y.
Try drawing a simple graph of the equation to see where the domain lies.
Mr F says: From your note below, I assume you're suggesting that the graph of y = f(x) = 3x+4 be drawn. That being the case, it's worth noting that this graph will make it clear what dom f and ran f are. This is useful since and .