The train crosses 90 km in 1 hour i.e. 3600 seconds
so in 36 seconds it will cover 90/3600 x 36 = 9/10 km.
now 9/10 km is the length of platform + length of the train
again length of the platform is double the train.
so (3 x length of train )= 9/10 km
so length of train is 9/10 x 1/3 = 3/10 km = 300 mtrs.
so the length of the platform is 600metres