easy.
given y = ax^2-6x-b
differentiate => y'=2ax-6
stationary point we put y'=0 => i.e 2ax-6=0 => solve for x => x=3/a
but we are given we have a stationary point at (3,2), this tells us that y'=0 for x=3 and y=2
Therefore x=3/a=3 => a=1
substitue a=1, x=3, y=-2 into your...