Answer :
[tex]x-\ length\ of\ river\ D\\\\
x+100-\ length\ of\ river\ C\\\\
x+x+100=5560\\\\
2x+100=5560\ \ \ | subtract\ 100\\\\
2x=5460\ \ \ | divide\ by\ 2\\\\
x=2730miles\\\\
2730+100=2830miles\\\\
River\ C\ is\ 2830\ miles\ long\ and \ river\ D\ is\ 2730\ miles\ long.[/tex]