(1/(5a) + 1/(7a))*a^2/4 = (7 + 5)/(35a)*a^2/4 = 12a^2/(35a*4) = 3a/35 =
= 3*7,7/35 = 3,3*7/(5*7) = 3,3/5 = 6,6/10 = 0,66
(3-3d)/6 = 4
3-3d = 24
-3d = 24-3
d = 21/(-3)
d = -7
4a - 4c + ac - a² = 4*3,5 - 4*(-1,5) + 3,5*(-1,5) - (3,5)² = 14 + 6 - 5,25 - 12,25 =
= 20 - 17,5 = 2,5