What is the location of the vertex of the parabola defined by f(x) = −x2 + 8x − 21 and in what direction does the parabola open?

the formula of a slope:

from a table we have

substitute:

the slope of f(x) is -1.

the graph of funcfion g(x) is a horizontal line. the slope of a horizontal line is equal 0. therefore the slope of g(x) is 0.

-1 < 0