Answer:
110.25
Step-by-step explanation:
Consider the square ...
(x +a)^2 = x^2 +2ax +a^2
The added constant (a^2) is the square of half the x-coefficient:
a^2 = ((2a)/2)^2
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The added constant needs to be the square of half the x-coefficient:
c = (-21/2)^2 = 441/4 = 110.25
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x^2 -21x +110.25 = (x -10.5)^2
