Q:

How do I multiply an input variable?

How do I multiply the number that is put into my input using Javascript?
This is a total beginner question - you are a good person for helping.
html
<input type="number" id="height" value="0" />

javascript
$(document).ready(function() {

    function display_height() {
        var new_height = $("#height").val() * 3);
        $("new_height").show();
    });
});

A:

There is no show method on a string. Try alerting your height instead:
$(document).ready(function() {

    function display_height() {
        var new_height = $("#height").val() * 3;
        alert(new_height);
    }
    display_height();
});

Additionally, you had minor problems like mismatched parentheses and braces, which I have corrected.
What I suspect you actually want is for the user to be able to multiply input, in which case you need a button:
<input id="multiply" type="button" value="Multiply!">

and a click handler:
$('#multiply').click(display_height);

You can see a demonstration here: http://jsfiddle.net/4wgaE/1/
To parse a string to a number, use parseFloat like this:
var new_height = parseFloat( $("#height").val() ) * 3;

