y = -x² + 9
the x-intercepts are x = - 3 and x = 3 hence the factors are
(x + 3 ) and (x - 3 )
polynomial is the product of its factors
y = a(x - 3 )(x + 3) ← ( a is a multiplier )
from the end behaviour the parabola opens down and is a maximum and so the value of a is negative
y = -(x - 3)(x+3) = - (x² - 9 ) = - x² + 9