Question

Two plane mirrors are inclined at angle 70 . A ray which incident on first mirror at angle of incidence x , comes out of the combination after reflection from the mirrors, parellel to the first mirror find x?

Answer 1

THETA = b 

angle of incidence = b = reflection angle. 
Look at the angles in the triangle formed by the reflected ray and the two mirrors. 

angle of incidence on the 2nd mirror = 90 - [180- 70 - (90-b) ] = 70 - b 
angle between the reflected ray and the mirror (2nd) = b+20. 

as it is parallel to the 1st mirror :   b+20 = 70

b=50

Answer 2

X = 70 degree............