Suppose we place Source S at the center C of curvature of concave mirror CM. Then the real image is formed at C itself. Then there is no role of plane mirror in this. So in this case S is placed at 50 cm from CM.
If plane mirror is to be involved then, the Source S is placed between the Focus F (25 cm) and C. So the rays form an image beyond the position of plane mirror M. That is rays reflected from concave mirror CM trying to converge behind the plane mirror M, get reflected again. The relation we have here is that , the distance of the image (to be formed by concave mirror) behind plane mirror is equal to the distance of the image in front of plane mirror.
Let V (positive value) be the distance of formation of image from concave mirror, if there is no plane mirror. Then (V - 200) will be the distance of this image behind plane mirror. Then the real image will be formed after reflection from plane mirror M at (V-200) infront of M. So actual image is at 200 - (V-200) = 400 - V cm from the concave mirror.
That is the source is placed at u = 400 - V cm from CM.
sign convention : u = - (400-V) v = - V
f = - 25 cm
1/v + 1/u = 1/f = -1/25
1/(400 - V) + 1/V = 1/25
400 / (400 V - V²) = 1/25
10,000 - 400 V + V² = 0
V = 200 + 100√3 cm and 200 - 100√3 cm -- ignore this as it is less than 200 cm
V = 373.20 cm
hence u = 400- V = 26.80 cm