The length sum goal in the thread Bruce linked above can be useful for more complex situations,
but if you know that the pulleys stay fixed and one end is the driver then things can be simplified.
Because the distance between the top 2 points stays fixed, you know that any change in the length of the segment on the left results in the negative of that change to the length of the segment on the right. So you can measure the distance and adjust the rest length like this:
simple_pulley.gh (16.7 KB)
edit - if you’re on Mac, you might need this for the grab tool.