Put a green sphere at the end of the bottom string.
Put a green sphere at the end of the middle string.
Switch green color to white color.
Switch white color to red color.
Collapse the bottom string using a red sphere.
Add a red sphere to the middle of the middle string.
Collapse that string with a red sphere.
Collapse the upper string with a red sphere.
Collapse the resulting string on the left using a red sphere.