The template has a fundamental flaw, in that it is leading you towards code that will not work if a single insert/update statement inserts or updates more than one row. Last time around, you got a D for doing the better/more efficient faster solution. So perhaps you will get a D if you wrote the code to handle multi-row inserts as well. If that is the case, sorry to say, you are between a rock and a hard place.
Really, the teaching assistant who created the template perhaps should take the class himself/herself.
Assuming you want to go with the flawed template, your "PUT IN YOUR CODE HERE" should have something like this:
IF LEN(@aVIN) = 17 AND RIGHT(@aVin,4) LIKE '[0-9][0-9][0-9][0-9]'
IF ( CAST(RIGHT(@aVin,4),INT) >= 1990 )