Question

9. triangle abc has vertices a(0,0), b(0,4), c(3,0). find the orthocenter of triangle abc.

Answer 1

http://www.mathopenref.com/constorthocenter.html
This website has always been a huge help to me anything Geometry related, so check this out, and you should get it down pretty easily. And, for your convenience, I made the triangle so you can print it if you would like. :)