the question is y=2x^2+16x-1

the book gave the incorrect answer of x=b/2a=16/2(2)=4 and that the axis of symmetry is x=4.

This parabola crosses the x axis at the 2 points (can be found from the solution equation of quadratic equation) x1,2= -4 + - sqrt(66)

The center of symmetry lies at midway between these two points of crossing which is xm=(x1+x2)/2=-4

There is also direct formula x=-b/(2a)=-16/(2*2)=-4

So x=4 is incorrect - you are right !

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