The height of an arch is modeled by the equation y=-x^2-6x+16. which of the following gives the width of the arch at its base?
the y-coordinate of the vertex
double the x-coordinate of the vertex
the y-intercept of the equation
the difference between the zeroes

we have

using a graph tool

see the attached figure      

The vertex of the quadratic equation is the point

That means that the maximum height of an arch is equal to

The zeroes of the function are the points

That means that the width of the arch at its base is equal to difference between the zeroes

therefore

the answer is

The width of the arch at its base is equal to the difference between the zeroes