Answer:
£130
Explanation:
Assuming I understood your question correctly, you have a television which costs in full, £1290 including all applicable VAT and tax. You paid £900 up front, which lowers your overall cost to pay it back down to £390. The rest you pay over the course of an unknown time period in equal payments.
In a real-world scenario, we would take interest into account but you provided none so lets assume this is a utopia where credit needs no interest.
In which case we do simple division, of 390/3 which is a nice round 130.
Simple! You pay £130 per payment.