This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 定 though, so here's a mnemonic to help you:
If you're having trouble with the calculation of the amount on your bill, just call Jourm (じょう). He's really great at numbers and he will be able to figure out the numbers in his head really fast.
Can I have the check, please?