In an example the MATIC:RUNE pool has 16,000 MATIC and 1,000 RUNE. The TUSD:RUNE has 3,300 TUSD and 10,000 RUNE. Based on this information alone, THORChain can know what 1 MATIC is worth in TUSD.
Here's the formula to work out the MATIC price ($/MATIC):