Question

The graph below shows the height of a tunnel f(x), in feet, depending on the distance from one side of the tunnel x, in feet:

Answer 1

The complete question in the attached figure
the correct vertex is (50,75)

we have that
x intercepts at ordered pairs (0, 0) and (100, 0)
The vertex is at (50, 75)

we know that
the general equation of a parabola is
y=ax²+bx+c

for x=0--------> y=0
so
c=0

for x=100-------> y=0
so
0=a*100²+b*100-------> 0=a*10000+b*100-------> 0=100*a+b
b=-100*a-------> equation 1

for x=50---------> y=75
so
75=a*50²+b*50--------> 75=a*2500+b*50-------> 1=33.33*a+0.67*b
1=33.33*a+0.67*b----------> equation 2

using a graph tool----------> to resolve the system 

the solution is 
a=-0.3
b=2.97

the equation is
y=-0.03x²+2.97x

Part A:
1) What do the x-intercepts and maximum value of the graph represent? 
The x-intercepts represent the points at the ground, so with that you have the width of the tunnel at ground level is 100 ft - 0 ft = 100 ft.
The vertex represents the maximum height, which is at the center of the tunnel. So, you know that the maximum height is at x = 50 ft and it is 75 ft

2) What are the intervals where the function is increasing and decreasing, and what do they represent about the distance and height? 
The increasing interval is (0,50), and the decreasing interval is (50,100).
That represents that the height increase from the left border (x = 0) until the center of the tunnel (x = 50 ft), reaching a height of 75 ft, and starts to decrease toward the right border until the ground level (x = 100).

Part B:
3) What is an approximate average rate of change of the graph from x = 15 to x = 35, and what does this rate represent?
Find the y -coordinate for x = 15 and for x = 35.

for x=15
y=y=-0.03x²+2.97x------> y=37.8 

for x=35
y=y=-0.03x²+2.97x------> y=67.2 

Then the average rate of change is equal to [change in height] / [change in x] = [67.2-37.8] / [35 - 15] = 29.4 / 20 = 1.47
1.47 > 0
the number is positive means that the height is increasing in that interval