What we know:
(Also,
)
What we need to solve:
(This is equal to
)
How you have to subtract
with
to get:

Thus, the answer would be:

Two lines that are parallel have the same slope. Thus, find the slope of the line (J) thru (4,2) and (5,1), and that will be the answer to your question.
2-1 1
m = --------- = ------ = -1 (answer)
4-5 -1