You have a 420
dinghy, not a Laser. The partners (where the mast goes through the
deck, or what would be deck) are cracked. You will need
fiberglass cloth tape and
epoxy resin (West Sytem, etc.) to fix this. Take the mast out. Sand, file, or grind the crack area to expose a bit more of the surface all around so that the new reinforcement has a good “grip” on the old material. Cut a thin piece of the
fiberglass cloth and wrap it around the crack, once around, and wet it with
epoxy so that it turns clear. You do not want any air spaces or bubbles in the cloth. While it is still wet, wrap another piece of fiberglass cloth, a little wider this time, over the first one. Wet this with epoxy the same way, and then wrap a still wider third layer, overlapping the second one, and wet that out too. The top layer should probably be about 10cm across. Try to get the epoxy to fill in the weave so that you can’t see any raised thread pattern. When it has cured you can sand it down smooth and if needed,
paint it. It would be worth looking to see if there are any Utube videos about this procedure in Dutch.