The missing factor is found as follows:
- we have to find two monomials that, when multiplied together, gives:

and, when added together gives:

Now, such two monomials are: 15s and 8s
since:

and:

Therefore, we can rewrite the left-hand side of the equation as:

We can now find the missing factor by factorization, as follows:

Therefore, the missing factor is: (s + 3)