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The 'x' coordinate of the vertex will be found by   - b / 2a          a = 4    b = -6    c = 15

   plug this value into the equation to find the 'y' coordinate.....  

Vertex: $[\dfrac{-b}{2a},f(\dfrac{-b}{2a})]$

you already know that a = 4, b = -6, c = 15

all you have to do from here is plug in the values!