How do you measure sq.ft. in a room that's not square?

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I am standing in a doorway that is 3 ft wide. From there a wall goes right at a 45 degree angle for 10 ft. The left wall also goes to the left at a 45 degree angle also for 10 ft. Both far ends of the 10 ft wall are 12 ft. apart.

I need to know(learn) how to calculate the square footage of rooms like this. Please don't just put the answer, I need to learn how to to it. Any help would be greatly appreciated.