def get_curve(pt0, a, n):
    """Given a, n, and point pt0, returns the Elliptic Curve that pt0 is on.

    Args:
        pt0 (tuple(int, int)): Point (x0, y0).
        a (int): Parameter a of the Elliptic Curve.
        n (int): Modulus.

    Returns:
        tuple(int, int, int): (a, b, n) representing the
        Elliptic Curve y**2 = x**3 + a*x + b (mod n).
    """
    x0, y0 = pt0
    b = (y0 ** 2 - x0 ** 3 - a * x0) % n
    return (a, b, n)