What is the axis of symmetry of the function f(x)=-(x+9)(x-21)

Carry out the mult.:  f(x) = -[x^2 - 21x + 9x - 189] 

Combine like terms:  f(x) = -[x^2 - 12x - 189]
Eliminate the brackets [   ]:      f(x) = -x^2 + 12x + 189
Identify coefficients a, b and c:   a= -1, b=12, c=189

The equation of the axis of symmetry is    x = -b/(2a), which here equals

x = -(12)/[2(-1)], or     x = 6

This is also the x-coordinate of the vertex.  Plug x=6 into the original equation to calculate the y-coordinate.