As far as I know, both answers are right. Since 6 and 2 are constants, c4 and c3 "absorb" them. Then when you find a particular solution, it doesn't matter the way it's written, the constants will adequate to the right result. For example, let's say you find your first constant from your solution and it's 6, then you'll get 6/6 * x^3. If you find the constants using the textbook solution you'll find that the first constant is equal to 1.