bonjour
aire = longueur * largeur
longueur = 4 + a
largeur = 8
aire = 8 ( 4 + a ) = 8 a + 32
8 a + 32 = 100
8 a = 100 - 32
8 a = 68
a = 68/8 = 8.5
a = 8.5 cm
