If the vertex of a parabola is (2,-9) and another point on the curve is (5,-18). what is the coefficient of the squared expression in the parabola's equation?

Parabola form : (y-k) = a(x-h)², where h and k are the vertex(h , k)
The vertex is given (2 , - 9), the equation of the parabola becomes:
y+9 = a(x-2)²
But we know this curve passes through (5, - 18)
Plug the respective values of x & y in the equation:
-18 +9 = a(5-2)² → - 9 = 9a & a = -9/9 and a= - 1