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.
![x + 90 = 120](/tpl/images/2520/6214/7d3da.png)
![x - 90 = 120 - 90](/tpl/images/2520/6214/18e24.png)
![x = 30](/tpl/images/2520/6214/7815c.png)
Step 2: Check if solution is correct.
![30 + 90 = 120](/tpl/images/2520/6214/c11bc.png)
![120 = 120](/tpl/images/2520/6214/1571c.png)
Therefore, m∠1 = 30 degrees.