bonjour,
2 x a + 5 x a = 2 a + 5 a = 7 a
6 x a + 5 x a = 6 a + 5 a = 11 a
2 x 3 + 5 x 3 = 3 ( 2 + 5 ) = 3 x 7 = 21
2 x 4 + 5 x 4 = 4 ( 2 + 5 ) = 4 x 7 = 28
2 x 6 + 5 x 6 = 6 ( 2 + 5 ) = 6 x 7 = 42
2 x 5 + 5 x 5 = 5 ( 2 + 5 ) = 5 x 7 = 35
voilà continue, tu isoles le facteur commun, donc celui qui revient