Answer
$p=3$
Work Step by Step
Since $p$ divides both $a$ and $a+3$, $p$ must divide $a+3-a = 3$. Since $3$ is prime, the only prime that divides it is $3$, so $p = 3$.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.