To the right is a serieshelpmate in seventeen by Thomas Rayner Dawson (published in Fairy Chess Review, 1947).
An effective way to solve long serieshelpmates such as this is to envisage a position in which Black could be checkmated, and then to work out how such a problem could be reached.
The piece on a2 has to be a rook, since if it is a queen or bishop, it would cover the b3 square, and if it was a knight, it would check the king.
The black king needs to walk half-way over the board to avoid the newborn rook giving check and then back to a1.
(As usual, chess problems with unintended multiple solutions are considered flawed; they are often said to be cooked.)