You can just expand it like i was a variable instead of a special number, and use the fact i^2 = -1 occasionally to reduce the power.
(4−5i)(5+5i)(2−7i)=(20−25i+20i−25i2)(2−7i)=(20−5i−25i2)(2−7i)=(45−5i)(2−7i)
Can you continue from here, doing what I just did?