How do I justify my answer?

Your answer would be yes, they would be parallel.

We can justify this using co-interior angles, which state that 2 angles subtended by parallel lines add up to 180°

We can check this with both angles 3x & x, or 2x & 90°.
If AD and BC are parallel, then 3x + x = 180, and since you already found the value of x we can substitute this in:
3(45) + (45) = 180
135 + 45 = 180
180 = 180

So the co-interior angles do add up to 180°, which means that AD and BC are parallel.

I hope this helps! Let me know if you have any questions :)