Find the equation of the parabola?

Clearly you have a maximum at x=4. Also you have a line through the two given points with equation y= -4.5x+29.5. Since this is the first derivative of the quadratic, The general form of the quadratic transforms to y= -4.5x^2 + 29.5x + c. Substituting one of the points in the equation produces c=-78. So your equation is Y= -4.5x^2 + 29.5x -78.