Look at the picture.
The formula of an area of a triangle:
[tex]A_\Delta=\dfrac{1}{2}ab\sin\alpha[/tex]
In the equilateral triangle, the measures of all angles are the same and equal to 60°.
Therefore your answer is:
[tex]\dfrac{1}{2}aa\sin(60^o)=\dfrac{1}{2}a^2\sin(60^o)[/tex]
