Ответ:

1) cab + ca = 100c + 10a + b + 10c + a = 11a + b + 110c

2) abc + bca = 100a + 10b + c + 100b + 10c + a = 110a + 110b + 11c

3) ab9 + 7a = 100a + 10b + 9 + 70 + a = 101a + 10b + 79