<?php
$numbers = array(5, 19, 45, 6, 7, 21);
$operators = array('*', '+', '-', '*', '+');

$mathString = '';
foreach ($numbers as $key => $value) {
    $mathString .= $value;
    if ($key < count($operators)) {
        $mathString .= $operators[$key];
    }
}

preg_match('/= ([0-9\.]+)/', file_get_contents('http://www.google.de/search?q='.urlencode($mathString)), $matches);
echo $matches{1};