The base angles of an isosceles triangle are equal
The base angles are 15 degrees, while the vertex angle is 150 degrees
The base angles are given as:
Base = (6a- 3) and (a + 12)
So, we have:

Collect like terms


Divide both sides by 5

Substitute 3 for a in Base = (6a- 3),



So, the base angle is 15 degrees.
The vertex angle is calculated using:

So, we have:


Hence, the vertex angle is 150 degrees
Read more about isosceles triangles at:
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