An isosceles triangle is a special type of triangle that has two equal sides. Consequently, the two base angles are also equal. Since the interior angles of a triangle should sum up to 180°, the general equation should be
180° = 2*Base angle + Vertex angle
Let the base angle be x. Thus, the vertex angle is equal to
Vertex angle = 4x - 6
Substituting,
180 = 2x + 4x - 6
x = 31°
Vertex angle = 4(31°) - 6
Vertex angle = 118°