Area with trigonometry

[languid oxide]

A rectangle is inscribed in an equilateral triangle whose perimeter is 30 cm.
a.)Express the area A of the rectangle as a function of the length x of the rectangle.
b.)Determine the dimensions of the rectangle with the largest area.

[trail ore]

The area of the rectangle is base times height, right?

[old karma]

A = root 3/2 (10 - x ) * x

maybe