To find the total number of postcards Lara buys after 3 days of vacation, we can substitute \( d = 3 \) into the equation \( p = 2d \).
Plugging in the value:
\[ p = 2(3) \] \[ p = 6 \]
Therefore, after 3 days of vacation, Lara will have bought a total of 6 postcards.