we have


Convert into vertex form
Group terms that contain the same variable, and move the constant to the opposite side of the equation

Complete the square. Remember to balance the equation by adding the same constants to each side


Rewrite as perfect squares

-------> equation in vertex form
This is the equation of a vertical parabola open down
The vertex is a maximum---------> 
therefore
the domain of the function is the interval---------> (-∞,∞)
Domain is all real numbers
The range of the function is the interval --------> (-∞,4]
Range is all real number less than or equal to 
Using a graphing tool
see the attached figure to better understand the problem