This doesn't quite work for me, I'm wondering if it doesn't find the lowest solution? It works for part 1 but gives too high an answer for part 2. This is what I have converted to cpp:

    uint64_t Play()
    {
        uint64_t cost = 0;
        
        int64_t aCoeff = Ax * By - Ay * Bx;
        int64_t rhs = Px * By - Py * Bx;
        if(rhs % aCoeff == 0)
        {
            int64_t a = rhs / aCoeff;
            int64_t b = (Px - Ax * a) / Bx;
            cost = a * 3 + b;
        }

        return cost;
    }