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as you know, the canonical form of a quadratic polynomial is

f(x) = a(x-b)^2 +c

and x=b is the symetric axe of its graph. c is its max/min value.

if x1 is a root of f(x1) and x1 <=b, then the other root x2= b+b-x1

in our case, b=5,x1=2, so x2=5+5-2=8

REALLY ELEMENTARY!