12x³-1 ( =5³x³-1) = {(5x)³-1} =(5x-1)³ Then use the difference of cube formula a³ - b³= ( a - b )(a²+ ab + b²) a=5x , b=1 a³-b³=(5x-1){(5x)²+(5x*1)+(1)²} Neglect minus sign in coeff of 1 when multiplying by 5x in (ab) Therefore =(5x-1)(25x²+5x+1) answered