Solving a problem in physics section 34 point 74 optics

74. When illuminated with monochromatic light Fresnel mirror (λ = 600 nm) from a narrow slit S on the screen, spaced at a distance a = 2.7 m from the line of intersection of the mirrors, interference fringes are observed which width b = 2,9 mm. The light source is located at a distance r = 10 cm from the line of intersection of the mirrors. Determine the angle between the mirrors.