first you need to make a line that is perpendicular to one side. The diagram is shown below. then the length of the line that is perpendicular to the triangle is
[tex] \frac{ \sqrt{3}x }{2} [/tex]
then the area of the equilateral triangle equals to
[tex] \frac{1}{2} height \times base[/tex]
so the area would be
[tex] \frac{ \sqrt{3} {x}^{2} }{4} [/tex]
