Answer:
12
Step-by-step explanation: it’s right
We have that

we know that
Is the equation of a vertical parabola open down
so
the vertex is a maximum
step 1
convert the equation of the parabola in the vertex form

the vertex (h,k) is the point (0,36)
Part a) The point on the graph where the height of the tunnel is a maximum is (0,36)
Part b) The points on the graph where the height of the tunnel is zero feet is when y=0
so
for y=0

the points are (-6,0) and (6,0)
see the attached figure
There are an infinite amount of equations that pass through a single point. You must provide the slope or y-intercept to get a single equation.
Hope that clarifies.