Line CA is a straight line, meaning it adds up to 180°
Line BE splits the line into two supplementary angles, because when the two angles are added together they will equal 180°
Using the rule of supplementary angles, we can then make the equation 3x + 8x + 15 = 180
Now, simplify the equation by combining like terms
11x + 15 = 180
To solve, isolate x
11x + 15 = 180
11x = 165
x = 15