(3^-1X^3)^2 x 12X^5

my work: 3/2X^6 x 12X^5= 36/2=18X^11 but the answer was 4/3X^11

3^-1 = 1/3, so you have

(x^3/3)^2(12x^5) = (x^6/9)(12x^5) = 4/3 x^11