Domain

For the equation to be undefined, the denominator must equal 0, because anything divided by 0 is undefined. 

Thus, we have the equation: $y^2-4y+4=0$, where we have to solve for y.

To solve for y, use the quadratic formula: $y = {-b \pm \sqrt{b^2-4ac} \over 2a}$ , because the quadratic is in the form $ax^2+bx+c=0$

Can you do it from here?

y^2 - 4y + 4  factors as    (y -2)^2

So

(y -2)^2 = 0

So  when y = 2, the denominator   = 0  and  the expression is undefined