In 2011, a Welsh club participated in the Premier League for the first time after Swansea City gained promotion. The first Premier League match to be played outside England was Swansea City's home match at the Liberty Stadium against Wigan Athletic on 20 August 2011. In 2012–13, Swansea qualified for the Europa League by winning the League Cup. The number of Welsh clubs in the Premier League increased to two for the first time in 2013–14, as Cardiff City gained promotion, but Cardiff City was relegated after its maiden season.

Answer this question, if possible (if impossible, reply "unanswerable"): After which city gained promotion did a Europa club join the Premier League?
unanswerable