[填空题]
Let $f(x)=a x^{2}-24 x+c$ , for $x \in \mathbb{R}$ , where a, c $\in \mathbb{Z}$ .
1. A horizontal line, L , intersects the graph of y=f(x) at x=1 and x=7 .
1. Find the equation of the axis of symmetry of the graph of y=f(x) .
2. Hence show that a=3 .
2. The equation of L is y=6 . Find the value of c .
