Besoin d'aide sur l'exercice 3)

a/ (x+3)(y+7) = xy+7x+3y+21 
b/ (x+5)(y-6)= xy-6x+5y-30 
c/ (x-4)(y+8)= xy+8x-4y-32
d/ (x-7)(y-2) = xy-2x-7y+14
e/ (2x+5)(y-9)= 2xy-18x+5y-45
f/  (3x-1)(4y+5)= 12xy+15x-4y-5