For what $a$ and $b$ is  $9x^4-12x^3+28x^2+ax+b$ a perfect square? If $9x^4-12x^3+28x^2+ax+b$ is a perfect square, find the value of $a$ and $b$. 
This is one of my past year examination's questions, some help on it?
(The answer for this problem is $a=-16$, $b=16$.)

$9x^4-12x^3+28x^2+ax+b=(3x^2+cx+d)^2=9x^4+6cx^3+(c^2+6d)x^2+2cdx+d^2$
So $c=-2, d=4$ and $a=2cd=-16, b=d^2=16$
