solve the polynomial by factoring

Question

(I know i can factor out 3x^3, but again idk what to do after that) thanks a bunch!

-12x^5 -15x^4 + 21x^3=0

Answer 1

-12x^5 -15x^4 + 21x^3
= -3x^3 (4x^2+5x-7)
you can always use the quadratic formula. Since the roots are
(-5±√137)/8
you know that it cannot be factored using rational coefficients