The intercept are the number of each of the pots used when no soil was put in the other pot.
When no soil was put in the large pots, then 120/5 = 24 small pots will be filled.
When no soil was put in the small pots, then 120/12 = 10 large pots will be filled.
THerefore, the points labelled are (24, 0) and (0, 10)