Bonjour,
a)
-3/5 * 7/7 = -21/35
4/7 *5/5 = 20/35
20/35 - 21/35 = -1/35
-2/5 * 7/7 = -14/35
-2/7 * 5/5 = -10/35
-14/35 - 10/35 = -24/35