Let x = Danielle, x + 5 = her sister, and .5x = her brother.
x + x + 5 + .5x = the sum of their ages.
Then we simplify to get 2.5x + 5 = the sum of their ages.
Zeroes of a function are the values of x when f(x) = 0
So to find the zeroes of the function from the graph search for the points whose y-coordinates = 0
The y-coordinates of the point = 0, if the points lie on the x-axis
That means the zeroes of the function are the points of intersection between the graph and the x-axis
let us see that in the graph
I will draw it and post it here
From the graph
The graph intersects x-axis at points (-7, 0) and (-2, 0)
Then the zeroes of the function are (-7, 0) and (-2, 0)
Let us make the table
x f(x)
-2 0
-1 6
0 14
1 24
2 36
It would increase the average, just by a bit.