Inscribed angles, substitution property of equality
Step-by-step explanation:
We know it cant be Central angles because its a quadrilateral so that's 2 answers out. Then you have to substitute each value to end up with 2 ⋅ m∠CFE + 2 ⋅ m∠CDE = 360°, it doesnt make sense to add to both sides so the answer is <u>Inscribed angles and substitution property of equality.</u>