The body of the constitution and the first 10 amendments are the main sources of:

The body of the constitution and the first 10 amendments are the main sources of constitutional law. Constitutional law involves rights that are described in the federal and state constitutions. These include rights provided to citizens as well as to governments. Oftentimes disputes about constitutional law are decided by the supreme court. These decisions also contribute to constitutional law.

Use the formula: (n-2)×180, where n is the number of sides(18-2)×180= 28800, now this is the full exterior angle of 18-gon so you divide it by 18.28800÷18=160