m∠ACB + m∠ECA = 180° (because they are supplementary angles)
m∠ACB = 65° (given).
Plug in 65 for m∠ACB.
65 + m∠ECA = 180
Isolate m∠ECA. Subtract 65 from both sides
m∠ECA + 65 (-65) = 180 (-65)
m∠ECA = 180 (-65)
m∠ECA = 115
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m∠ECA = 115
115° is your answer
hope this helps