What is the unknown measure (the question mark)

The unknown measure is 6 because it is an equilateral triangle, and equilateral triangles have equal angle measures and side lengths.

Hope that helps ;)

m∠5 + m∠3 = m∠1   is true.

m∠5 + m∠6 = 180 is true

m∠2 +m∠3 = m∠6   is true

m∠2 +m∠3 +m∠5 = 180   is true