Q:

Using a button to display the outcome of a coin flip Javascript

I am trying to make a simple coinflip that outputs a certain image, for example in this case it is ctflip and tflip based on whether its true or false.  When I press the button I want ti to run the coinFlip function to generate the number and then display the correct image. 
    <! DOCTYPE html>
    <html>
    <body>
    <h1>My Web Page</h1>
    <p>My paragraph.</p>
    <p id="flipDisplay"></p>
    <button onclick="coinFlip()"+"display()">Flip the Coin!</button>
    <script>
    function coinFlip() {
    return (Math.floor(Math.random() * 2) == 0);
    }
    function display() {
    if (coinFlip()) {
        document.write('<img src="./tflip.png">');
    }
    else {document.write('<img src="./ctflip.png">');
    }
    }
    </script>
    <! --<a><img src=javacript:flip></a> -->
    </body>
    </html>

A:

Your syntax for calling two functions is incorrect.   Only calling display should work fine here, because display calls coinflip itself. 
   <! DOCTYPE html>
    <html>
    <body>
    <h1>My Web Page</h1>
    <p>My paragraph.</p>
    <p id="flipDisplay"></p>
    <button onclick="display()">Flip the Coin!</button>
    <script>
      function coinFlip() {
        return (Math.floor(Math.random() * 2) == 0);
      }
      function display() {
        if (coinFlip()) {
          document.write('<img src="./tflip.png">');
        } else {
          document.write('<img src="./ctflip.png">');
        }
      }
    </script>
    <! --<a><img src=javacript:flip></a> -->
    </body>
    </html>

The proper syntax for calling two functions is onClick="first();second();" but it is often better to just call one function that calls others as needed.