speed of the train is ?? please help?

think of it a little differently. maybe even draw it out, if it helps you

the sound from two bullets will always be a constant distance apart and a constant time apart

the train will have a steeper line (closer to vertical) the slower it is.... if it were going at zero speed, it would hear the bullets at 10 1/2 mins apart

if it were going very, very fat, the line would be less steep (closer to horizontal) and (ignoring problems with doppler shift) they would sound closer together

The wavefront of the sound of the first shot travels at 330 m/s towards the train, arriving at some time t = 0 secs as noted on the train.

If the train was stationary the second shot would have been heard at t = 630 as heard on the train.

But in fact the train had been moving towards the gun position at v m/s and so the wavefront had less distance to travel. The result was the second shot was heard only 600 seconds later. That means the train was nearer by the amount sound travels in 30 secs which is 3,300 metres.

[That explained why distance covered by sound in 30 seconds is equal to distance covered by train in 10 minutes as you asked]

The train took 600 seconds to travel 3,300 metres so speed v = 5.5 metres/sec

If you want that in km/hr multiply by 3.6, so it is 19.8 km/hr