现在学习范丽安中级微观现代观点，现在有一道题目很不明白，请求帮助，问题采纳后还有重谢！

the price elasticity of demand for a certain agricultural product is constant (over the relevant range of prices)and equal to --1.5,the supply elasticity for this product is constant and equal to 4,Originally the equilibrium price of this good was 15 per unit,then it was discovered that consumption of this product was unhealth,the quantity that would be demanded at any price fell by 11%,The percent change in the long-run equilibrium consumption of this good was???

中文意思大概就是 一个特定农产品的价格需求弹力是负1.5，价格供给弹性是4，开始市场均衡价格是15快，然后发现这个产品对身体有害，所以需求下降11%在任何一个给的价格下，试问在这个产品在长期的均衡中得消费的改变比例是多少？？
这道题目的答案是--8% （负8%）

请求达人给点建议帮助，，不胜感激！！！！！