First find the slope.
We know these two points are in the line: (0,8) and (6,5)

The slope is -1/2. The y-intercept can be found when x = 0. And we know that the point (0,8) is on the line and x = 0, therefore the y-intercept is 8.
Plugging this into the slope-intercept form which is
y = mx + b, where m is the slope and b is the y-intercept, we get:
y = (-1/2)x + 8