x5 + 24x4 - 12x3 by 6x
Your equation is wierd. Can you try reformatting it maybe?
Generally the "by" means like a square, such as saying 8 by 8 we assume 8 * 8 (I may be wrong for this guy).
\((x^5 + 24x^4 - 12x^3) * 6x = 6x^6 + 144x^5 - 72x^4\)