Wikipedia has a long article about this problem: https://en.wikipedia.org/wiki/Pell%27s_equation

I finally got my solution to work. It was taking too long on the last two cases. Turns out that manual calculation was faster than using python's fractions library.

Is there a way for you to see the time taken by your code in each test case? I am a noobie and I like to know how fast the computer can crunch the numbers.

why this problem has been tagged as "Easy"??

when D = 61, it looks like the minimal x is 1766319049. It is really too crazy to iterator until this digit