A coordinate grid is shown below:
2 answers:
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You have the intercepts and x-coordinate of the vertex correct. There appears to be a math error in the calculation of the y-value of the vertex.
f(7/8) = (4*7/8 -7)*(7/8) -15 = -49/16 -15 = -289/16 = -18.0625
f(7/8) = (4*7/8 -7)*(7/8) -15 = -49/16 -15 = -289/16 = -18.0625