How do I solve system of equations by subsitution

Given: ac and db are diameters. m∠3=

In a school 2/3 of the students study a language of those who study a language 2/5 study french

Which approach to the theory-and-research cycle does the following sequence reflect: (1) specific observations suggest generalizations, (2) generalizations produce a tentative theory, (3) the theory is tested through the formation of hypotheses, and (4) hypotheses may provide suggestions for additional observations?