What is the maximum value of the function y = −3x2 − 15x + 9?

Y = -3x² - 15x + 9
The graph of this equation is an n-shape. It will go up to a maximum point and then arc back downwards.
In order to find this point, there are two ways I can think of, I will show the one I prefer:
First, take out the coefficient of the x² terms:
y = -3(x² + 5x - 3)
Now, CTS (complete the square):
y = -3((x + 5/2)² - 25/4 - 3)
y = -3(x + 5/2)² + 111/4
Maximum Point: (-5/2, 111/4) or (-2.5, 27.75)

Idon't know that language