The <em><u>correct answe</u></em>r is:
An exterior angle is supplementary to the adjacent interior angle.
Explanation:
An exterior angle of a triangle and its adjacent interior angle form a linear pair. This means they form a straight line, and a straight line has a measure of 180°. Since they combine for a total of 180°, this means they are supplementary.
The mystery number is 16 because 16 - 6 = 10.
we know that
The equation in vertex form of a vertical parabola is equal to

where
(h,k) is the vertex of the parabola
and the axis of symmetry is equal to the x-coordinate of the vertex
so
-----> equation of the axis of symmetry
In this problem we have

Convert to vertex form
Group terms that contain the same variable, and move the constant to the opposite side of the equation


Complete the square. Remember to balance the equation by adding the same constants to each side


Rewrite as perfect squares
the vertex is the point 
therefore
<u>the answer is</u>
The axis of symmetry is

see the attached figure to better understand the problem
3x-21=57
3x=21+57
3x=78
Divide by 3
X=26