Since the y-value for each point is the same, 5, the line is horizontal. Therefor the equation of the line is y = 5.
To really prove this you can look at the slope m = Δy/Δx and the point-slope formula y - y1 = m(x - x1) to really prove it to yourself.
For the slope of (6, 5) and (3,5) we calculate this by finding the difference in y-values divided by the difference in x-values.
Δy = 5 - 5 = 0
Δx = 3 - 6 = -3
m = 0/-3 = 0, so the slop is zero. Using the formula y - y1 = m(x - x1) you can pick either point you like, let's use the (6, 5) point so
y - 5 = 0(x - 6)
y - 5 = 0
y = 5
It checks out.