Classes Product.php undefined variable problem

[ukbaz]

Hi am getting following error in server error logs:
Undefined variable: $tax in /var/www/vhosts/xxxxxxxxx.co.uk/httpdocs/classes/Product.php on line 1247.

Line 2247 is :
$attribute_price = $usetax ? $result[‘attribute_price’] : ($result[‘attribute_price’] / (1 + (($tax ? $tax : $result[‘rate’]) / 100)));

Any ideas why this is – have moved tax code to the bottom as in example above? Have attached Product.php class

Thanks
Baz

Product.php

[ukbaz]

Can no-one help with this? thanks
Baz