The perimeter of an isosceles triangle is 18 decimeters, and the length of one side is 7 decimeters. What is the length of the base of this triangle?
一个等腰三角形的周长是18分米，一腰长为7分米，这个三角形的底长是多少分米？
<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">18 - 14</gadget>
<output>4</output>

<result>4</result>