The equation of a line passing through the origin and perpendicular to the line 7x - 3y + 4 = 0 is:
7x - 3y + 4 = 0
3x + 7y = 0
3x - 7y + 4 = 0
7x - 3y = 0
The equation of a straight line passing through the origin and perpendicular to the straight line 2x + 3y - 7 = 0 is ______.