What is the axis of symmetry of f(x) = 2x² - 12x + 7?

The axis of symmetry of $ax^2+bx+c$ is $x=-\frac{b}{2a}$. Here $a=2$ and $b=-12$, so $x=-\frac{-12}{2\cdot2}=\frac{12}{4}=3$, choice D. Check by completing the square: $2(x^2-6x)+7=2(x-3)^2-18+7$, so the vertex is at $x=3$. Choice A ($-3$) loses the sign of $b$. Choice B ($12$) is $-b$, with no division by $2a$. Choice C ($6$) divides by $2$ rather than by $2a=4$. The sign matters: a negative $b$ places the axis to the right of the $y$-axis.