So the equation of this parabola is a quadratic which means there is a handy formula to find the line of symmetry. The general quadratic form is ax^2 +bx +c, so for our equation:
a=1
b=-12
c=7
The handy formula to find the line of symmetry of a parabola like this is:
x= -b/2a, where x equals the line.
With this we can determine that our line is 12/2 which equals 6.
Our line is at x=6