{f-3}{f-e}-{6+f}{6++f}

(f-3)(f-3)-(6+f)(6+f)
= (f^2-6f+9)-(f^2+12f+36)
=(-6-12)f+9-36
=-18f-27


why lots of people calculate it wrong.......

(f-3)(f-3)-(6+f)(6+f)
=f-3xf-3-6+fx6+f
=f^2+9-36+f^2
=f^2+f^2+9-36
=f^4-27

Apply (a + b)^2 = a^2 + 2ab + b^2 and (a - b)^2 = a^2 - 2ab + b^2,
(f-3)(f-3)-(6+f)(6+f)
= (f^2 - 6f + 9) - (36 +12f + f^2)
= f^2 - 6f + 9 - 36 - 12f - f^2
= -18f - 27

Remark: Do carefully when you extract the brackets.