从数据库填充的下拉菜单中更新输入字段。

I have a graphing calculator I am working on. I want to user to save any valid equation they write. They can choose from the saved equation by selecting a drop down menu. The problem I have is that when I click on any equation in the dropdown menu, the input box is not updated. Is there a way I can update the input box by clicking on any equations? This is what I have: I have a graphing calculator I am working on. I