The temperature is 30 degrees Celsius.
First you sub 348 for v in the equation which gives you 348= 20_/273+t. Then you divide both sides by 20 which gives you 17.4=_/273+t. Then you square both sides to get rid of the square root which gives you 302.76=273 +t, subtract 273 from both sides and you end up with t=29.76.
Hope this helps :)