To do this, we take the coefficient of the x term, which is 4, divide it by 2, and then square it. This gives us $(4/2)^2 = 2^2 = 4$. We then add this to both sides of the equation to maintain equality. This gives us:
$$
x^{2}+4 x + 4 = 21 + 4
$$
which simplifies
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