The curve is a second degree equation in x which tells us it is a parabola

 

Find the minima

And hence find the co-ordinates of the vertex of the parabola......

 

Now the equation of the axis would be

  x= abcissa of the vertex

The graph is now symmetric about the vertex and because the coefficient of x^2 is +ve, it would be upper parabola

If it were negative, it would have been upside down, rest all remainin the same........