y - 2x = 3
need the slope
y = 2x + 3
slope = 2
need perpendicular
perpendicular slope = -1/2
need equation of line with slope = -1/2, passing through point (1,2)
point-slope form
y - y1 = m (x - x1)
y - 2 = (-1/2) (x - 1)
y - 2 = (-1/2)x + 1/2
y = (-1/2)x + 5/2
need standard form
(1/2)x + y = 5/2