Y=x^2-4x+1

Y=2x-3

Y=1/x^2-4

The domain of a function is the allowable set of "input" or argument values for which the function is defined. In all three cases you are looking for allowable values of x. So think about what value of x (if any) are not allowed for each function because y would then be undefined. Post back with your attempt at answering this and we'll give some more help if you need it.