Step-by-step explanation:
Supplementary angle are angles that have their sum to be 180°. According to the question, since ∠1 and ∠2 are supplementary and ∠2 and ∠3 are supplementary, then;
m∠1+m∠2=180° .... 1
m∠2+m∠3=180°.... 2
In order for Daniella to get m∠1 and m∠3 to both equal the same expression, he will need to take the difference of the two equation as shown
(m∠1-m∠3)+(m∠2-m∠2) = 180°-180°
(m∠1-m∠3)+0 = 0
m∠1-m∠3 = 0
Add m∠3 to both sides of the equation
m∠1-m∠3+m∠3 = 0+m∠3
m∠1 = m∠3 Proved!