Bonjour
a - b
= 7/3 - 10/21
= (7 x 7) / (3 x 7) - 10/21
= 49/21 - 10/21
= 39/21
a x b
= 7/3 x 10/21
= 70/63
= (7 x 10) / (7 x 9)
= 10/9
2a + b
= 2 x 7/3 + 10/21
= 14/3 + 10/21
= (14 x 7) / (3 x 7) + 10/21
= 98/21 + 10/21
= 108/21
= (3 x 36) / (3 x 7)
= 36/7.