You obtain

At this point, we're going to make a sign table:

x...| ....... 0 .........2 .........9 ...........

f(x)|........0...........|..........0..........

Take a test point for example thus:

x...| ....... 0 .........2 .........9 ...........

f(x)|....-...0....+.....|...-.....0.....+......

Therefore the solution of this inequality is: