The white king is on
and the last few moves were
c2-c4 b4xc3 e.p.; Kb3xc3.
Either the WK is on b3 (in which case it's white's move and black's last must have produced this position in which the WK is in check twice), or the WK is somewhere else (in which case it's black's move and white's last must have produced this position in which the BK is in check from the WB).
Let's consider these possibilities in order.
Suppose the WK is on b3. There's no possible last black move that can have created both checks on the WK, so this isn't possible.
the WK is not on b3 and black is in check right now. If white's last move was with the bishop then it was along the a4-d1 diagonal and black was already in check, which is impossible. So white's last move was with the king, and before that move black was not in check. So white has just moved his king from b3.
If that move wasn't a capture then we have exactly the same position we just ruled out. So the WK has just moved from b3, necessarily to a3 or c3 since the other squares adjacent to b3 are either occupied or attacked, capturing a black piece. On a3 or c3 that piece can't be obstructing either of the two checks the WK is in, so it must have moved from somewhere that did.
This is no good yet, because
we need black's last move to have created two checks, so black's move must also have removed a white piece that was blocking a check, without recreating that block. There's only one way to do that! Black must have made a capture en passant, and there's only one possible such capture.