Answer:
Choice A (x=9)
Explanation:
There are 180 degrees in a triangle, as seen in the picture; the triangle shown is an equilateral triangle meaning all angles are the same; therefore, each side has to be 60 degrees. Therefore the answer we plug into 5x+15 must simplify to 60. So:
5x+15
plug in 9
= 5 (9) + 15
= 45 + 15
= 60