So we start with the mean, which is given as 13.5. Write that down
like this:

13.5

Now we take the standard deviation, which is given as 1.5, and
multiply it by 2 (because it says "2 standard deviations") and we
get 1.5 x 2 which equals 3, which we write as 3.0.

Now since we want to make the mean, which is 13.5, less by
2 standard deviations, we know that to make something less,
we must subtract from it.  So underneath the 13.5. we write 3.0.

13.5
 3.0

Next we subtract:

13.5
-3.0
----
10.5  

Now we notice that choice a) is 10.5, so that's the
correct answer.  Didn't take a rocket scientist, did it?

Edwin