Anything other than making x - 3 = 0 will make x - 3 have a value. No matter what happens (x - 3)^2 is positive. There is a minus sign outside the brackets which makes (x - 3)^2 negative.
That means that something is always taken away from 5 unless x = 3. Try and find any value that will make a number bigger than or equal to five that isn't x = 3.
By the way, I'm assuming the question is y = 5 - (x - 3)^2
