The only way for the substitution effect to be positive would be for our consumer to consume fewer than R_O packs of noodles when she finds herself on the compensated budget constraint. As we can see this would make her worse off than her original bundle since such bundles lie below her original indifference curve (shaded area indicates bundles that make her worse off than her original bundle). She would never buy fewer than R_O packs of noodles in this setting so the substitution effect is negative.