This study is testing a medicine called **cabozantinib** with chemotherapy to see if it helps patients with a bone cancer called **osteosarcoma**. **Osteosarcoma** is a type of cancer that starts in the bones. **Cabozantinib** is a drug that blocks signals that help cancer cells grow and form new blood vessels. The study will compare patients who get this drug with those who only get usual chemotherapy, which includes three drugs: methotrexate, doxorubicin, and cisplatin. **Chemotherapy** is a treatment that uses strong drugs to kill cancer cells. The study has different parts, including a phase to test if adding cabozantinib is safe and another phase to see if it works better than usual treatment alone. - **Study Length**: The study has several cycles, each lasting 28-35 days, with multiple cycles for treatment and maintenance. - **Visits Needed**: Patients will have regular hospital visits for chemotherapy treatments and check-ups, including scans and blood tests. - **Risks**: There could be side effects from the drugs, and patients must be able to swallow pills and not have other serious health issues. Patients under 40 with newly diagnosed osteosarcoma may join, but they must not have had prior chemotherapy for this cancer.