Answer
(a,a$\sqrt 3$)
Work Step by Step
The height of an equilateral triangle, which would be the y coordinate of the vertex in this case, is equal to half of the base times the square root of 3. Therefore:
y=.5(2a)$\sqrt 3$
y=a$\sqrt 3$
The vertex id equidistant from the other two vertices so its x coordinate is equal to half of the base.
x=.5(2a)
x=a
Vertex=(x,y)=(a,a$\sqrt 3$)
