Then, ∠ b = 30.
Therefore, ∠ BAC =180-30-30 =120
If BA⊥DA is known, then ∠ bad = 90.
Therefore, ∠ DAC = 120-90 = 30
So, AD=CD.
Let ad = cd = x.
In Rt△ABD, ∠ b = 30.
Then BD=2AD=2x.
Therefore, BC=BD+CD=2x+x=3x.
BC=6 is known.
So 3x=6.
Then x=2.
Therefore, AD=2 and BD=4.
So, from Pythagorean theorem, we get: AB? =BD? -Advertising? = 16-4= 12
Therefore, AB=√ 12=2√3.