The graph of which function has an axis of symmetry at x = 3?

f(x) = x2 + 3x + 1

f(x) = x2 – 3x – 3

f(x) = x2 + 6x + 3

f(x) = x2 – 6x – 1

mark this and return

You do this by converting to vertex form a(x - b)^2 + c   where the axis of symmetry  is x = b

the correct function to have a symmetry x = 3 will be the one which includes -6x
which is the last one 

x^2 - 6x - 1 =  (x - 3)^2 -9-1

the last option is the correct one

The answer should be c.