Question 288881
I don't quite understand what you mean by solve. When we solve an equation we look for the roots or zeros.
It is already in the form to know the x's 2 and 6 when y=0
All you have to do is graph it.It is a parabola.
I have no idea how you got 21x^2-36 but it is wrong. the -36 is right
y=-3(x-2)(x-6) 
y=-3(x^2-8x+12)
y=-3x^2+24x-36
The vertex (high point) is at (4,12)
It opens down because of -3x^2