Create a new Intent name: addTwoNumbers.

Under Training phrases type: add 5 and 6 and Change the parameters name: number to a & number1 to b.

Then go to Fulfillment session under intent. and Enable webhook call for this intent.

Now go to Fulfillment menu undersidebar and Enable the Inline Editor.

Define intentMap.set('YourIntentName', FunctionsName) Functions. For example in this intent I'm using

    intentMap.set('addTwoNumbers', AddNumber);

Create a function named 'addTwoNumbers'.

  function AddNumber(agent){
    var x =agent.parameters.a;
    var y =agent.parameters.b;
    var z =x+y;
    agent.add('sum of '+x+' and '+y+' = '+z);
  }

Click on Deploy Button and Test it.