from tkinter import *
from math import *
def setwindow(root):
    root.title('Окно программы')
    root.resizable(False, False)
    w = 800
    h = 600
    ws = root.winfo_screenwidth()
    wh = root.winfo_screenheight()
    x = int(ws / 2 - w / 2)
    y = int(wh / 2 - h / 2)
    root.geometry("{0}x{1}+{2}+{3}".format(w, h, x, y))

def equation():
    global a
    global b
    global c
    global result
    x1 = str(float(b.get() * -1) + float(sqrt(b.get() ** 2 - float(a.get() * c.get() * 4) / float(a.get() * 2))))
    x2 = str(float(b.get() * -1) - float(sqrt(b.get() ** 2 - float(a.get() * c.get() * 4) / float(a.get() * 2))))
    result.config(text="x1 = " + x1 + "; " + "x2 = " + x2)

root = Tk()
setwindow(root)

label_main = Label(root, text='ax^2 + bx + c = 0', font='tahoma 20')
a = Entry(root, font='Tahoma 16')
c = Entry(root, font='Tahoma 16')
result = Label(root, font='Tahoma 18')
but = Button(root, text='Вычеслить корень уравнения ', font='Tahoma 20', command=equation)
labela = Label(root, text='a', font='Tahoma 16')
labelb = Label(root, text='b', font='Tahoma 16')
labelc = Label(root, text='c', font='Tahoma 16')

label_main.place(relx=0.5, rely=0.01, anchor='n')
a.place(relx=0.5, rely=0.1, anchor='n')
b.place(relx=0.5, rely=0.2, anchor='n')
c.place(relx=0.5, rely=0.3, anchor='n')
labela.place(relx=0.3, rely=0.09, anchor='n')
labelb.place(relx=0.3, rely=0.19, anchor='n')
labelc.place(relx=0.3, rely=0.29, anchor='n')
but.place(relx=0.5, rely=0.5, anchor='n')
root.mainloop()