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*it has only been one year since I took my thermo course. I did poorly and I forget most of it. Nonetheless, I would proceed as follows*...
Just past the water tables in your text appendix, you should find some r134a tables.
So you need to find the pressure that corresponds to -20. Make sure youre dealing on the right hand side of your saturation curve because youre nearly a saturated vapor, not nearly saturated liquid.
Then use your ideal gas equation (with an R for r134a) and back calc to your initial temp. Depending on where you are in the course, you might need to make a general compressibility adjustment.
again, take that advice with a grain of salt.
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