Supplementary means they add up to 180, so you can write an equation:
3x + 2x - 25 = 180
5x - 25 = 180
+ 25
5x = 205
÷ 5
x = 41
I hope this helps!
The formula to find the measure of an interior angle is

, where

represents the number of sides in the polygon. In this problem,

would represent

.
Let's plug in the value for


Let's simplify that.

Now we know that the <span>measure of an interior angle of a regular three-sided polygon is 60</span>°.
Your answer would be B!Let me know if you have any questions regarding this problem!
Thanks!
-TetraFish