The lenght of AB side is 4.
To be a parallelogram, the length of parallel sides must be the same. This is a propierty of parallelograms.
In a parallelogram:

Now all we have to do is solve the equation:

Now we know the value of Q and we can replace it in AB=4Q-8

The lenght of AB is 8.
To be sure we can check what happends when we replace Q in CD=Q+4

With this, AB=CD and we know we have the correct answer