1/3x - 1/7x + 1/7y + 1/2y = (1/3 - 1/7)x + (1/7 + 1/2)y = (7/21 - 3/21)x + (1/7 + 3/7)y = 4/21x + 4/7y
1/3x - 1/7x + 1/7y + 1/2y = (1/3 - 1/7)x + (1/7 + 1/2)y = (7/21 - 3/21)x + (1/7 + 3/7)y = 4/21x + 4/7y