Answer:
To get 9d^2 + 6d + 1 from (3d+1), you would need to multiply by 3d+1 again.
Expanding (3d+1)(3d+1), you get:
9d^2 + 3d + 3d + 1
Simplifying, you get:
9d^2 + 6d + 1
Therefore, you need to multiply (3d+1) by (3d+1) or (3d+1)^2 to get 9d^2 + 6d + 1.