a.
The equation of the axis of symmetry of a parabola [tex]y=ax^2+bx+c[/tex] is [tex]x=-\frac{b}{2a}[/tex].
[tex]h(x)=-x^2+6x \\
a=-1 \\
b=6 \\ \\
\hbox{the axis of symmetry:} \\
x=-\frac{6}{2 \times (-1)} \\
x=-\frac{6}{-2} \\
x=-(-3) \\ x=3[/tex]
The equation of the axis of symmetry is x=3.
b.
[tex]x=3 \\ \\
h(x)=-x^2+6x \\
h(3)=-3^2+6 \times 3 \\
h(3)=-9+18 \\
h(3)=9[/tex]
At the axis of symmetry the arch is 9 feet high.