first let's find the sign of the product ( u - 6)( u - 3)
the two roots are 3 and 6
when 0 < x < 3 and 6 < x < 9 ===> ( u - 6)( u - 3) > 0 ===> | ( u - 6)( u - 3) | = ( u - 6)( u - 3)
when 3 < x < 6 =========== > ( u - 6)( u - 3) < 0 ====> | ( u - 6)( u - 3)| = - ( u - 6)( u - 3)
the second integral should be splitted in three parts:
hope this helps