//function to display or hide a given element

function showHideItems(myItem, myButton){

  //this is the ID of the hidden item
  var myItem = document.getElementById(myItem);
  
  //this is the ID of the plus/minus button image
  var myButton = document.getElementById(myButton);

  if (myItem.style.display != "none") {

      //items are currently displayed, so hide them

      myItem.style.display = "none";

      swapImage(myButton,"plus");
  }
  else {

      //items are currently hidden, so display them
      myItem.style.display = "block";

      swapImage(myButton,"minus");
  }
}

//function to swap an image based on its current state
function swapImage(myImage, state) {

    if (state == "minus") {

      myImage.src = "design/minus-red.png";
    }

    else {

      myImage.src = "design/plus-green.png";
    }
}