5.

(3x + 2)^2 - (3x - 1)^2 = (3x+2- (3x - 1))*(3x + 2 + (3x - 1)) = 3*(6x + 1)

6.

(b - 5)(b + 5)(b^2 + 25) - (b^2 - 9)^2 = (b^2 - 25)(b^2 + 25) - (b^2 - 9)^2 = b^4 - 25^2 - (b^4 - 18b^2 + 81) = 18b^2 - 706

При b=-1/3 значение равно 18*1/9 - 706 = 2 - 706 = -704