It is known that the perimeter of an isosceles triangle is 42 cm, and its waist length is three times the base length. What is the length of the base of this triangle in cm?
已知一个等腰三角形的周长是42厘米，它的腰长是底长的3倍，这个三角形的底边长是多少厘米？
<gadget id="calculator">3 + 3 + 1</gadget>
<output>7</output>

<gadget id="calculator">42 / 7</gadget>
<output>6</output>

<result>6</result>