Question

If the slope is -2 and point A is (2,4) what would point B be?

Answer 1

Any other point of the line: y = -2x + 8

Answer 2

So what would the point of point B be?

Answer 3

There are actually infinite number of possible point B here, because if point A and point B lie on a line with a slope of -2, then two different points on that line have can be connected with a line of slope equal to -2. (I hope that's clear ^^;)
But just to show how to get one point on the line, we use here the formula for slope:
m = (y2 - y1)/(x2 - x1)
where m is the slope, and (x1,y1) and (x2,y2) are points on the line.
Substituting,
-2 = (y2 - 4)/(x2 - 2)
-2/1 = (y2 - 4)/(x2 - 2)
Equating numerators and denominators,
y2 - 4 = -2
y2 = -2 + 4
y2 = 2

x2 - 2 = 1
x2 = 1 + 2
x2 = 3

Thus, point B can be (3,2).

If however, the slope is rewritten as 4/(-2), which is still equal to -2, equating,
y2 - 4 = 4
y2 = 8

x2 - 2 = -2
x2 = 0

Thus, point B can also be (0,8), and there are infinite solutions.

Hope this helps :)