g(x) = -x² + 10x - 30
the equation of a quadratic in vertex form is
y = a(x - h)² + k
where (h, k ) are the coordinates of the vertex and a is a multiplier
here vertex = (5, - 5 ), hence
y = a(x - 5)² - 5
to find a substitute the point on the graph (8, - 14) into the equation
- 14 = 9a - 5 ⇒ 9a = - 9 ⇒ a = - 1
y = - (x - 5)² - 5 = - (x² - 10x + 25) - 5 = - x² + 10x - 30
g(x) = - x² + 10x - 30 ← in standard form