D) construction of the angle bisector.
So if you have a triangle that is not equilateral the perpendicular bisectors would not work so you need angle bisectors.
I have drawn a diagram
The green is the angle bisectors and the yellow the perpendicular bisectors.
You can see the problem straight away
15+(10-x)=15 would be your equation if that is what you are looking for.
Answer:
x= 55
Step-by-step explanation:
The two angles add to 180 degrees since they form a straight line
2x+70 =180
Subtract 70 from each side
2x+70-70=180-70
2x= 110
Divide by 2
2x/2 = 110/2
x= 55