Question 49908
For addition and subtraction of polynomials (what we have here), we just combine like terms.  "Like terms" are terms with the same variable to the same degree.  So -6X3 and 14X3 are like terms in your first problem.  Keep in mind that the way you are writing X3 and X2 can more accurately be written x^3 and x^2, but I will use your notation for simplicity.
So for part a, we get 8X3 + 10X2 - 20X - 4.
Now, for part b, the subtraction sign gets distributed, giving (13X4 - 8X3 + 2X2 ) + (-5X4 + 3X3 - 2X2 + 6).  Now do this the same way as part a.