For a 13-year-old weighing 42 kg, the usual labeled dose is:

This age group uses the adolescent/adult dose, not a weight-based dose. The FDA labeling specifies 600 mg every 12 hours for patients aged 12 years and older. FDA prescribing information

However, synovitis itself is not an indication for linezolid. It may be used only if there is confirmed or strongly suspected bacterial joint infection, such as MRSA or resistant Gram-positive infection, and should be directed by a pediatric infectious-disease or orthopedic clinician. A swollen painful joint with fever, inability to bear weight, or reduced movement needs urgent assessment for septic arthritis, often including joint aspiration and drainage.

For courses beyond about 1-2 weeks, clinicians usually monitor CBC/platelets and assess for neuropathy and drug interactions, particularly SSRIs and other serotonergic medicines.