Key fact for two successive reflections from inclined plane mirrors
If a ray reflects successively from two plane mirrors inclined at an angle $\phi$, then the net change in direction of the ray depends only on $\phi$ and is equal to $2\phi.$ Here, $\phi = 75^\circ.$ So the angle through which the direction of the ray turns is $2\times 75^\circ = 150^\circ.$

Meaning of total deviation
In optics, the total deviation is the angle between the original straight-line direction of the incident ray (produced forward) and the final emergent ray.
Since the ray has turned by $150^\circ$, the deviation is the reflex angle associated with this geometry: $\delta = 360^\circ - 150^\circ = 210^\circ.$

Check with given extra data
The given reflection at $M_2$ is $30^\circ$; this helps fix the actual path but does not change the standard result that two reflections from mirrors inclined at $75^\circ$ produce a net turning of $150^\circ$, hence total deviation $210^\circ$ in the convention used here.

Final answer $\boxed{210}$