there is no difference, except that for pseudorandomness one with generator knowledge can predict the next value, for "truly random" nobody knows the generator.
Yes, this is a good question.
But first to clarify that c is just a number, not a point. Therefore the more correct question, why we need 4 numbers (coordinates of 2 SPECIFIC points) and only 2 numbers (m and c).
The answer is quite simple: with equation y=mx+c, you define a line running...
They are independent in a sense that one can draw different straight lines using different m and c.
But if one has a fixed point (y1, x1) and c, then m is defined, same for (y1,x1) and m, then c defined.
As in original question:
It is obvious that grzz meant a straight line, thus 2 points DEFINE a unique line. The statement about "the same line" although may be true but misleading and irrelevant in the context.
m and c are not independent.
Consider equation y=mx+c for points (y1,x1) and (y2,x2).
Solving it you will have:
m=(y2-y1)/(x2-x1); c=(y1+y1)/(2m(x2+x1)).
the similar thinking can lead you to understanding that to define a line you need just TWO points, and a number of coordinates for each point...