You are wrong because you are using the same method for two situations that are not the same.
Have a brief read here. When a number is positive its two's complement representation corresponds to its "normal" binary representation, so when you want to convert a number (sign plus absolute value, any base):
- if it's positive you just convert its absolute value in binary form
- if it's negative you make the two's complement of its absolute value in binary form
When you want to come back things are just the same: you can tell the sign looking at the first bit: if it's zero the number is positive, if it's 1 it's negative. That's why the book tells you that these are 12 bits numbers, without that information you would have been right for both your answers.
So, to extend the bits passing through a sign-magnitude representation:
- if the msb is 1 convert the number as you did, add zeroes, go back to two's complement
- if the msb is 1 convert the number, i.e. keep it as is, add zeroes, go back to two's complement, i.e. keep it as is
And that's it. As Ignacio writes it happens that it's enough to just extend the MSB to the left (you can proof that starting from the formulas on wikipedia), but that's just a quick way to do things and does not explain why you were wrong. Keep his advice as a way on how to check your calculations... And (in the future) as the way to avoid errors.