Let x be the length of the train.
On the basis of the observer;
Speed of the train = x/6
On the basis of the bridge;
Total distance covered by any point of the train= 350+x
Speed = (350+x)/20
Equating the two expressions of speed;
x/6 = (350+x)/20
20(x) = 6(350+x)
20x = 2100+6x
(20-6)x = 2100
14x = 2100
x= 2100/14 = 150 m
Speed = x/6 = (350+x)/20 = 150/6 = 500/20 = 25 m/s.
Therefore,
Length of train = 150 m
Speed of train = 25 m/s