Answer
$\phi = 0.23 ~\lambda$
Work Step by Step
The amplitude of the resulting wave is $~~2y_m cos \frac{\phi}{2}$
We can find the phase difference $\phi$:
$2y_m cos \frac{\phi}{2} = 1.5~y_m$
$cos \frac{\phi}{2} = \frac{1.5}{2}$
$\frac{\phi}{2} = cos^{-1} (\frac{1.5}{2})$
$\phi = 2~cos^{-1} (\frac{1.5}{2})$
$\phi = (2)(0.7227~rad)$
$\phi = 1.4454~rad$
We can express this in units of wavelengths:
$\phi = \frac{1.4454~rad}{2\pi rad} \lambda$
$\phi = 0.23 ~\lambda$
