Simple regular expression problem
You're almost there.

$((a+b+c)(a+b+c))^∗$ gives you strings of even length.

$((a+b+c)(a+b+c))^∗a$ gives you strings of even length plus an&nbsp;$a$.  That is, odd-length strings that contain at least one&nbsp;$a$ and end with an&nbsp;$a$.

Now, we're told that there are no $b$s after the last&nbsp;$a$. And there are certainly no&nbsp;$a$s after the last&nbsp;$a$, because that's what "last" means! So there can only be $c$s after the last&nbsp;$a$.

So, the answer is going to have to look something like $((a+b+c)(a+b+c))^∗ac^*$. That's not quite right, because it doesn't force the whole string to be even-length and it does force the last&nbsp;$a$ to be in an odd-numbered position in the string (whereas, hint, it should be able to be even or odd).

I'm going to stop there because you showed enough in the question that I think you can finish this yourself, now.