In other words, y-2 goes in the first box and 5 goes in the second box.
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Work Shown:
y^2 - 4y - x + 9 = 0
y^2 - 4y + 9 = x
x = y^2 - 4y + 9
x = y^2 - 4y + 4 + 5 .... rewrite 9 as 4+5
x = (y^2-4y+4) + 5
x = (y-2)^2 + 5 .... apply the perfect square factoring rule
So we'll have y-2 go in the first box and 5 goes in the second box
note: One version of the perfect square factoring rule says (a-b)^2 = a^2-2ab+b^2.