30
Step-by-step explanation:
1. The exterior angle theorem states that the two opposing (in terms of the exterior angle) interior angles of a triangle are congruent to the exterior angle. This means that m∠1 (x) + 90 = 120. Now, we just have to solve for x.
2. (Solving)
Step 1: Subtract 90 from both sides.
Step 2: Check if solution is correct.
Therefore, m∠1 = 30 degrees.