Let d and d′ be the common differences of the two APs respectively.
Given: d=d′+13.
From the second AP: b43−b31=12d′, so −385−(−277)=12d′, giving −108=12d′ and d′=−9.
Therefore d=−9+13=4.
For the first AP: a78=a1+77d, so 327=a1+77(4)=a1+308.
Thus a1=19