The original number is 3.
I actually began with the guess-and-check method, but seeing as that won't always work, let's go over the formal way. To get the original number, you first need to determine how many times the number was squared.
To make it simple, let's use x to focus on the exponents. The number was squared 3 times, so x^2, x^2, x^2. Basically, you need to multiply. 2 * 2 * 2 = 8. So, now find the 8th root of 6561 (depending on the calculator, you can just input it). You should come up with 3. Let me know if this part confuses you.
To find the next 2 numbers, you just need to continue the pattern.
6561^2 = 43,046,721
43,046,721^2 = 1,853,020,188,851,841
To my knowledge, which means this could be wrong, they're both perfect squares. Since the number to get them both were whole numbers, they should both have a square root that equals a whole number.
)/6 roots
)/(2*3)