Here's a link to dividing long polynomials: Polynomial Long Division
Think of the problem as:
You will see that you'll get a remainder of 0, indicating that:
Ok, so the first term in the quotient is x^2. I just do not know where to go with that. Do you get x^3+(a+b)x^2 and how do you subtract that from x^3+0x^2. How do you ever get the a+b without the x so you can subtract the a^3+b^3 terms? I just do not see how you get this done.
When dividing, we always match the leftmost term of each line so that when subtracting, they disappear (those in colour):
LaTex is kind of hard to work with when it comes to long division but hopefully, you can see where to go from here. Treat all the terms with s and s as constants in front of .