import math
from Tkinter import *
tk = Tk()
t = StringVar()#Текст
a = IntVar()#Переменные, заданные пользователем
b = IntVar()
c = IntVar()
...
Entry(tk,
    textvariable = a,
    takefocus=False,
    ).pack()
Entry(tk,
    textvariable = b,
    takefocus=True,
    ).pack()
Entry(tk,
    textvariable = c,
    takefocus=False,
    ).pack()
def prog (a, b, c):
    D = (b*b) - 4*a*c
    x1 = ...
    x2 = ...
    otvet = x1, x2
but = Button(tk,
          text=k.decode("cp1251"), #надпись на кнопке
          width=15,height=2,) #ширина и высота
but.bind("<Button-1>",prog(a,b,c))
but.pack()
...
tk.mainloop()

Traceback (most recent call last):
  File "C:\Users\Сергей\Desktop\прог\test.py", line 44, in <module>
    but.bind("<Button-1>",prog(a, b, c))
  File "C:\Users\Сергей\Desktop\прог\test.py", line 37, in prog
    D = (b*b) - 4*a*c
TypeError: unsupported operand type(s) for *: 'instance' and 'instance'

import math
from Tkinter import *
tk = Tk()
t = StringVar()#Текст
a = IntVar()#Переменные, заданные пользователем
b = IntVar()
c = IntVar()
k = "Считать"#Кнопка
s = "ля-ля тополя"
t.set(s.decode("cp1251"))
Label(tk,
    textvariable=t,
    relief="groove",
    borderwidth=3,
    font=("Courier", 14, "bold"),
    justify=LEFT,
    width=80,
    padx=10,
    pady=20,
    takefocus=True,
    ).pack()
Entry(tk,
    textvariable = a,
    takefocus=False,
    ).pack()
Entry(tk,
    textvariable = b,
    takefocus=True,
    ).pack()
Entry(tk,
    textvariable = c,
    takefocus=False,
    ).pack()
a1 = a.get() #Возвращаем переменным числовые значения
b1 = b.get()
c1 = c.get()
с2 = IntVar()
c2.set(c1) #Даем объекту с2 числовое значение с1
Label(tk,
    textvariable=c2,
    relief="groove",
    borderwidth=3,
    font=("Courier", 14, "bold"),
    justify=LEFT,
    width=80,
    padx=10,
    pady=20,
    takefocus=False,
    ).pack()
tk.mainloop()()

import Tkinter
import string

def cash(event):
   x = string.atoi(Entry.get())
   y = x/20 + x/10
   Label ["text"] = "%s" % y

root = Tkinter.Tk()

Entry = Tkinter.Entry(root)
Entry.bind("<Double-Button>", cash)
Entry.pack()

Label = Tkinter.Label(root)
Label.pack()

root.mainloop()

try:x=int(Entry.get())
except ValueError:return