Respuesta :

-21x^3 - 8x^3 = -29x^3
-7/4x^(1/2) - 0 = -7/4x^(1/2)
-6x^(1/4)
-12x^(-1/2)
12x^(-1/4)
14x^4

Therefore, the correct option is the 2nd one.