Question

find the domain of the rational function
f(y) = y+4 / y^2 - 4y

Answer 1

I'm assuming that the entire expression "y^2 - 4y" is in the denominator.

You can't have a 0 denominator. What would make = 0?
Well, solve y^2 - 4y = 0
Factor out a y
y(y - 4) = 0
y = 0 or y = 4.
So the domain is all real numbers except y=0 and y = 4