We are given two points: (4, -8) and (8, 5)
General equation of a line: y = mx + c
First find m (the gradient). There is a formula for this: (change in y)/(change in x) = gradient
(5 - -8) / (8 - 4) = (13) / (4) = 3.25 = m
y = 3.25x + c
Now we need to find c (the y-intercept; the value of y when x=0).
Substitute in any one of the coordinates - let's use (8, 5)
(5) = 3.25(8) + c
(5) = 26 + c
c = -21
The equation of the line: y = 3.25x - 21
I think that this answer is correct - sorry if it seems rushed.
Hope this answer helps :)