Analytical Geometry help please!

I really hope someone can help with this.

I am afrikaans so please excuse my english names.

We have a project in Maths asking us the following:

In a rectangle ABC, where a line connects C directly with the basis with AB to form a 90 degrees angle(i believe its called a hightline, in direct translation).

http://img74.imageshack.us/img74/4767/gfnm1.png

They now want us to prove this(Why it forms a 90 degree angle) by using Analytical geomatry, can someone help out 'n bit please?