Here's what I learned: Angle BDC = 2x is an exterior angle of triangle ABD.
Hence, it is the sum of the two non-adjacent angles. This is the key rule I didn't know beforehand. You can plug in numbers to find out that this will always be true.
That is: angle BDC = angle BAD + angle ABD
Hence: 2x = x + angle ABD
Then: angle ABD = x
Once you know this, you know AD = BD because it's an isosceles triangle, and likewise for BD = BC. Thus, AD = BD = BC.
So (1) is sufficient.
