The answer is: " 3 " ; that is: " ∠ 3 " .
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From the image attached, " ∠ 3 " is complementary to " ∠4 " ; since "complementary angles", by definition, add up to "90° " (i.e. "completementary angles form a "right angle" ; and "∠ 3 " and "∠ 4 " , together" form a "right angle" (i.e. form a "90° angle").
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As such: " ∠ 3 " is complementary to "∠ 4 " .
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