You have a bunch of issues:
1. WordPress part:
Your function is hooked to wp_enqueue_scripts and should be like this:
function wpse238953_calculate() {
wp_enqueue_script('calculate', plugins_url('/js/calculate.js'), __FILE__), array('jquery'), false);
}
add_action('wp_enqueue_scripts', 'wpse238953_calculate');
Note the different spellings between wp_enqueue_scripts and wp_enqueue_script
2. The HTML/JS part:
Let’s add some id to the two input fields so that it’s easier to target them
<form name="form">
InputA: <input type="text" id="inputA" name="inputA" /><br />
InputB: <input type="text" id="inputB" name="inputB" /><br />
<input type="button" id="calculate" value="Calculate" /><br />
Your result is: <b><span id="result"></span></b>
</form>
Then rewrite your JS and use some jQuery(since you added it as a dependency):
jQuery(document).ready(function($){
'use strict';
$('#calculate').mousedown(function(){
var valueA = parseFloat( $('#inputA').val(), 10 ),
valueB = parseFloat( $('#inputB').val(), 10 ),
total = ( valueA + valueB ) * 10;
$('#result').text( total );
});
});
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