Respuesta :

Answer:

7r^-1/3r^3

Step-by-step explanation:

7r^2/21r^3

21r^3 ➡ 7r^3 × 3r^3

7r^2/7r^3×3r^3 ➡ 7r^2 - 7r^3/3r^3

➡7r^-1/3r^3