geometry

Rotate \(\triangle ADC\) so \(AD\) lies on \(DC\) and \(DC\) lies on \(DB\). 

 

Because of similar triangles, \(AD:DC = DC: DB = 4:6\). 

 

We now have this pair of ratios to solve for: \(4: 6 = 6: \text{__}\)

 

Can you solve it now?