def euler(f, x0, y0, xn, n):
    """
    Forward Euler's Method for a single linear equation.

    Parameters
    ----------
    f : function
        linear equation.
    x0 : float
        initial x value.
    y0 : float
        initial f(x) value.
    xn : float
        x-value at which to estimate f.
    n : integer
        number of iterations.

    Returns
    -------
    x_values : array
        list of x values.
    y_estimate : array
        list of f(x) values.
    """
    y_estimate = [y0]
    x_values = [x0]
    h = abs((xn - x0)/n)

    for i in range(n):
        y_estimate.append(y_estimate[i] + h*f(x_values[i], y_estimate[i]))
        x_values.append(x_values[i] + h)

    return x_values, y_estimate