Do it this way..may be it will help u to understand better ,
The concep behind fragmentation is dependent on MTU, and payload to be sent across that network's MTU.
So here we have MTU of 1500bytes ( 1st network)
Data to be sent 2800 bytes
Divide it multiples of 8, that to in such a way that after adding 20 bytes of (header) to the fragment we get <= MTU,
So here we have , 1480+20 and 1320+20,
Now for second network divide above fragments(only data part not IP header one) into multiples of 8,
1.) 1480 can be divided as
480+20, 480+20(IP PART), 480+20,24(4 bits appended to make remaining 20 data bits multiple of 8)+20
Same For 1320
480+20,480+20,64(same as above 4 bits appended)+20
Now add all the IP headers for second network..u will get 140 bytes...
Hope it helps!