Follow the reasoning here:

"Ten more than (something)" tells me that we're going to be 
adding 10 to the "something".

So let's see what the "something" is that we're going to add 10 to:

That's "(something) less than 12

"(Something) less than 12" means we're going to be subtracting something from 12.

So let's see what "the something" is we're going to be subtracting from 12:

That's 3x.

So let's subtract 3x from 12.  To do that we put 12 down first 
then subtract 3x from it, giving us:

12 - 3x

We said in the beginning we were going to add 10 to what we got.  
So let's add 10 to it.

12 - 3x + 10 

Edwin