It is given ,
Length of the train = 150 meter
Speed of the train =50 km/hr =$50*\frac{5}{18}$ m/sec
Time took to cross the bridge =45 sec
length of the bridge =x meters
We know that ,
distance =speed *time
=> $(x+150)=45*50*\frac{5}{18}$
=> x+150=625
=>x=475
so, length of the bridge =475 meters