To find the number of people who have watched the video after 3 days using the function \( y = 4(6)^t \), we can substitute \( t \) with 3:
\[ y = 4(6)^3 \]
Now calculate \( (6)^3 \):
\[ (6)^3 = 6 \times 6 \times 6 = 216 \]
Next, multiply that by 4:
\[ y = 4 \times 216 = 864 \]
Therefore, the number of people who have watched the video after 3 days is 864.