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y= the square root of ( (4-x)^2 - 5 )

I set what was under the square root greater than or equal to zero. and came up with one answer however, I don't understand why my book tells me to put (4-x) greater than or equal to square root of 5 and (4-x) less than or equal to - square root of 5. I'm only accustomed to setting up two equations like they did with absolute values... Thanks.