Explanation:
let's start with the point-slope form
y - y1 = a(x - x1)
where (x1, y1) is a point on the line, "a" is the slope.
the slope is the ratio
y coordinate difference / x coordinate difference
when going from one point on the line to another.
we got 2 points
(5, 11) and (9, 7).
when going from the first to the second
x changes by +4 (from 5 to 9).
y changes by -4 (from 11 to 7).
the slope (a) is then -4/+4 = -1.
I pick simply the first point (5, 11), and we get as point-slope form :
y - 11 = -1×(x - 5) = -(x - 5) = -x + 5
adding x and 11 to both sides gives us the requested equation :
x + y = 16