The perimeter of an isosceles triangle is 57 cm and the waist is 21 cm. What is the base in cm?
一个等腰三角形的周长是57厘米，腰是21厘米，它的底是多少厘米？
<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<gadget id="calculator">57 - 42</gadget>
<output>15</output>

<result>15</result>