Question e732b9c

Advanced Math - Equivalent expressions

$open parenthesis, a x plus 3, close parenthesis, times, open parenthesis, 5 x squared, minus b x, plus 4, close parenthesis, equals, 20 x cubed, minus 9 x squared, minus 2 x, plus 12$

The equation above is true for all x, where a and b are constants. What is the value of ab ?