DO NOT expand the second factor completely and then multiply the result by the first factor. That is WAY too much work!
The second factor, when expanded, will contain terms of x^12, x^7, x^2, x^-3, x^-8, x^-13, and x^-18.
The only place where we will get a constant term (x^0) when that factor is multiplied by (1-x^2+x^3) is when the x^3 in the first factor is multiplied by the x^-3 term in the second factor.
The coefficient of the x^-3 term in the expanded second factor is
Then the constant term in the completely expanded expression is
