Answer:
When adding polynomials you've got to combine all like terms
2u^3 + 7u^3= 9u^3 (They both are raised to the third with the same variable.)
6u^2 (Stays the same because there are no other numbers that are raised to the second)
-7u (Again there are no other numbers to combine this with)
2+4=6 (Add 2+4 as normal)
Now you put together all the numbers that are left for your final answer
9u^3+6u^2-7u+6
Hope this helps ;)