Answer:
[tex]\boxed{12u^3+8u^2-6u+9}[/tex]
Step-by-step explanation:
[tex](8u^3+8u^2+6)+(4u^3-6u+3) = 8u^3+8u^2+6+4u^3-6u+3[/tex]
Once
[tex]8x+4x = 12x, \text{ as } x = u^3[/tex]
[tex]8u^3+8u^2+6+4u^3-6u+3 = 12u^3+8u^2+6-6u+3 = \boxed{12u^3+8u^2-6u+9}[/tex]
You don't need to factor.
