def temp_metal_linear(x, a, b, c):
    """Return the value of a plane with the given parameters.

    Parameters
    ----------
    x : tuple of floats, length-2
        A tuple containing pairs of floats representing temperature and
        metallicity of a star.
    a, b, c : float
        Parameters for the equation. `a` is the offset, `b` is the constant
        for the linear term in temperature, and `c` the constant for the linear
        term in metallicity.

    Returns
    -------
    float
        The value of the function at the given independent variable values with
        the given parameters.

    """

    temp, mtl = x

    return a + b * temp + c * mtl