[Blindmath] Basic mathematical concept question

[Duong Tuan Nam]

Hello all,
I don't clearly understand some basic problems.
How do continuous and discontinuous function
work.
Why is the slope of a line is equal y over x?
Another is that why derivative of a funtion at a point is the
slope of the tangent line at that point of its graph?

I've searched on Google but most of the Math webpage result can't be read 
with Jaws on IE.
Can you show me some accessible webpage to read or help me explain these 
problems?
Thank you very much,
Nam