The Mysql query does not work with mathematical operators

Where is a problem because I try count values using sql queries:

(SELECT quantity FROM db WHERE no='998') this is fine

but (('500') - (SELECT quantity FROM db WHERE no='998')) // incorrect syntax near -

But I need to use constant 500. Where is problem

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How about this?

SELECT 500 - quantity
FROM db
WHERE no = 998;

A select statement needs to start with a select. In addition, numeric constants should not use single quotes (although that has no effect on whether the query parses or runs).