To add distances, lay them end-to-end in the same direction.
To subtract distance, lay the negatve one along the positive one so their ends lie at the same point.
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The attachment gives the general idea of the layout. You draw the negative distances on the same line. (Here, they're shown parallel for clarity.) You must set your compass or dividers to the distances given on your page.
It would be convenient to mark a beginning spot near the left end of each given line. I would do this exercise by marking the "a" distance on all the lines to begin with, then set the compass or dividers to "b" and add or subtract it the required number of times, then set the compass or dividers to "c" and add or subtract that in the two places required.
You must be careful not to disturb the distance setting of the compass or dividers after you set the desired length. Put one point of the instrument on the beginning mark, then use the instrument to make a mark where the end of the segment lies. Use this mark to add or subtract additional segments as required.
