My code doesn't work?

I am really new to python and just tried to make this Pythagorean theorem calculator for the sake of it and it works except for when I try and calculate b. I know having b isn’t necessary but I don’t know why the or doesn’t work. Please let me know how to fix this.
Thanks!

import time
import math
def repeat():
    print ("Welcome to the pythagoras calculator!")
    time.sleep(1.5)
    calculation = input ("Do you want to find a, b or c?\na/b/c\n")
    if calculation == ("a" or "b"):
        if calculation == "a":
            b = int(input ("What is b?"))
        elif calculation == "b":
            a = int(input ("What is a?"))
        c = int(input ("What is c?"))
        if calculation == "a":
            a = (math.sqrt(c**2 - b**2))
            print ("a is ", a, ". Thank you for using my calculator!")
        elif calculation == "b:":
            b = (math.sqrt(c**2 - a**2))
            print ("b is ", b, ". Thank you for using my calculator!")
    elif calculation == "c":
            a = int(input ("What is a?"))
            b = int(input ("What is b?"))
            c = (math.sqrt(a**2 + b**2))
            print ("c is ", c, ". Thank you for using my calculator!")
    else:
        print ("Please enter a valid letter next time")
while True:
    time.sleep(1)
    repeat()

This is not how you would check if calculation is equal to “a” or “b”. You must make two separate comparisons in the same statement. One for “a” and one for “b” compared to calculation using the equality operator ==.