Question 487059
You can draw angle bisector from the vertex, which you have been given. 
This will create two congruent triangles, since the triangle is isosceles.
The side opposite this vertex will be connected at its midpoint by the segment that is formed from the angle bisector.
You can then determine the side opposite the half of the vertex angle by the tangent function, tan 1/2 vertex angle = opposite length/adjacent length (height);
this will be half of your base.
The formula will be in terms of base:
2 ((tan 1/2 vertex angle ) * adjacent angle.)
Cleomenius.