I can find missing angle measures of a triangle by using angle properties to write equations.

INTERIOR ANGLE SUM

The sum of the measures of the interior angles of a triangle is 180°.

m∠1 + m∠2 + m∠3 = 180°

EXTERIOR ANGLE THEOREM

The measure of an exterior angle of a triangle is equal to the sum of the

measures of its remote interior angles.

m∠2 + m∠3 = m∠4

DIAGRAM DESCRIPTION

A triangle has interior angles labeled 1 (at the right base), 2 (at the top),

and 3 (at the left base). Its base extends to the right, forming exterior

angle 4 adjacent to interior angle 1. Angles 2 and 3 are identified as the

remote interior angles relative to exterior angle 4.