first step: find slope of line
2x - y = 1
2x = y + 1
y = 2x - 1
slope of line: 2
perpendicular to slope of line: - 1/2
now we want the equation of the line passing through (2, -1) with slope = -1/2
y - y1 = m (x - x1) (point-slope form)
y - -1 = (-1/2) (x -2)
y + 1 = (-1/2)x + 1
y = (-1/2)x