i will recheck this. i vaguely remember some method where you always use e^(some power) for the integrating factor, but i'm not sure if you did it correctly (i rarely remember formulas or routines verbatim, i just know how to work stuff out). i know the last line is wrong though, since 8x is the power of e, not 3x^4.
if you want to know how i got my integrating factor, ask