Answer
$\phi = 20^{\circ}$
Work Step by Step
Note that the light ray reflects off a mirror at the same angle as it strikes the mirror.
Thus the angle between the surface of the left mirror and the incident ray is $40^{\circ}$, and the angle between the surface of the left mirror and the reflected ray is $40^{\circ}$
We can find the angle between the surface of the right mirror and the incident ray:
$180^{\circ}-80^{\circ}-40^{\circ} = 60^{\circ}$
Then the angle between the surface of the right mirror and the reflected ray is $60^{\circ}$
We can find $\phi$:
$\phi = 60^{\circ}-40^{\circ}$
$\phi = 20^{\circ}$
