def linear_fct(x, a, b):
    """Return f(x) of the linear function f(x) = ax + b.

    :param x: (float) X-coord
    :param a: (float) slope
    :param b: (float) intercept

    """
    x = float(x)
    a = float(a)
    b = float(b)
    return (a * x) + b