List and function while and else

[doug2019]

Hi! I need code a list "d" to plot the s x t graph. The list "d" must have six itens too but if user input a value greater "a" in the list this value must be 0 in according to the equation "d=x*0". This code is a simplified example. Can someone help me?

import matplotlib.pyplot
a=int(input("a=")
b=int(input("b=")
c=[0,1,2,3,4,5]
d=[]
x=0
while (x<=a):
    d=b*(a-x)
    x=x+50
else:
    d=x*0
    d.append(x) 
matplotlib.pyplot.title("Average Speed")
matplotlib.pyplot.plot(c,d)
matplotlib.pyplot.xlabel("t [s]")
matplotlib.pyplot.ylabel("s [mi]")
matplotlib.pyplot.show()   

Note
When I run the code it displays the following error message:
AttributeError: 'int' object has no attribute 'append'