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!
