The slope of the line segment is (8 - 4)/(-5 - (-3)) = -2, so the slope of the perpendicular bisector is 1/2. The midpoint is (-4,6), so by point-slope form, the equation fo the line is y - 6 = 1/2(x + 4). Then y = 1/2*x + 8, so m + b = 1/2 + 8 = 17/2.