Question

Y=-x^2 + 11x-18 vertex

Answer 1

Be just a bit clearer, please:  "find the vertex."

You could either "complete the square" or use the "quadratic formula."

Here the coefficients are a=-1, b=11 and c=-18.  Because a is -, the curve opens downward and the vertex is on top, representing the function's maximum.

Use the quadratic formula with the coefficients given above.

        -b plus or minus sqrt(b^2-4ac)
x = -------------------------------------------
                           2a