Two Strategies
1. Make use of the formula for the axis of symmetry for ax²+bx+c, which tells you that line is ...
... x = -b/(2a)
... x = -16/(2·(-2))
... x = 4
2. Let a graphing calculator show you the vertex, from which the axis of symmetry is easily determined. (See the attachment.) The graph shows the line of symmetry is x = 4.
My Preference
My preference is for the graphing calculator solution (for problems with integer solutions). The one shown is quick and easy and involves no arithmetic. If the coefficients of the equation are not integers, then the formula version is preferred for an exact solution.
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Comment on vertex form
When you put the equation into vertex form, you find a constant for the location of the vertex that is essentially equivalent to -b/(2a). Doing that generally involves three steps: finding b/a, then dividing that in half to get b/(2a), then realizing that the actual vertex location is at the x-value that is the opposite of the one in the vertex-form equation, so is -b/(2a).
