The perimeter of an equilateral triangle is 21cm, the height is 4.5cm, what is its area in cm**2.
等边三角形的周长是21cm，高是4.5cm，它的面积是多少cm**2．
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">4.5 / 2</gadget>
<output>2.25</output>

<gadget id="calculator">7 * 2.25</gadget>
<output>15.75</output>

<result>15.75</result>