Question 18697
You can start with the formula for the slope of a line when you are given two points on the line. {{{m = (y2-y1)/(x2-x1)}}}.

The given points are: (5, -8) this is (x1, y1) and (3, y) this is (x2, y2)

{{{m = (y - (-8))/(3 - 5)}}} But m = -5
{{{-5 = (y + 8)/-2)}}} Multiply both sides by -2
{{{10 = y + 8}}} Subtract 8 from both sides.
{{{y = 2}}}

Check: 

{{{m = (2 - (-8))/(3 - 5)}}}
{{{m = (2 + 8)/-2}}}
{{{m = 10/-2}}}
{{{m = -5}}}