Smallest whole number coefficient?
You need to factor in the electrons that are being moved around

Chromium is doing this:
$\ce {Cr^{2+}_{(aq)} + 2 e- -&gt;  Cr_{(s)}}$

Aluminum is doing the reverse of this:

$\ce {Al^{3+}_{(aq)} + 3 e- -&gt; Al_{(s)}}$

Now, like a math equation flip around the aluminum reaction.
And balance out the electrons

$\ce {[Cr^{2+} + 2 e- -&gt;   Cr_{(s)}] \times 3}$

$\ce {[Al_{(s)} -&gt; Al^{3+} + 3 e- ] \times 2}$



$\ce {3Cr^{2+} + 6 e- + 2 Al_{(s)} -&gt; 3Cr_{(s)} + 2 Al^{3+} + 6 e-}$

Hope this helps