hello and welcome to brainly!
so in the hint it pretty much gives you the equation x=-b/2a
the standard form of a quadratic equation is f(x) = ax^2+bx+c
so we look at the equation we have and we can see that a=-2 and b=-12
we substitute this into the axis of symmetry equation
x= -(-12) / 2(-2)
simplify
x=12/-4
simplify further
x=-3
