For a parabola whose equation is
f(x) = ax<sup>2</sup> + bx + c
the x-coordinate of the vertex is -b/2a. Once you have the x-coordinate, substitute it for x in the equation, and evaluate to find f(x)....which is the y-coordinate of the vertex.
Your equation is
f(x) = 3x<sup>2</sup> + 18x - 3
a = 3, and b = 18. The x-coordinate of the vertex, -b/2a, is -18/2(3), or -18/6, or -3. Now substitute -3 for x:
f(-3) = 3(-3)<sup>2</sup> + 18(-3) - 3
Do the arithmetic on the right-hand side, and you'll have the y-coordinate of the vertex.
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