guess and check man, fortunately you only gotta check the things less than the square root of 841. I honestly do not know a better way to do it.
You could probably limit some of the possibilities by considering residues mod 10. Like to get a 1 for the ones place you know you are going to need something that will be 0+1, or 2+9, or 3+8 or 4+7 or 5+6, then figure out which integers squared give you those things and check those. Like I think nothing squared gives a ones digit of 2, 3, 7, or 8. So I think the only possible ways of ending in a 1 would be a 0+1 or a 5+6. So you need to look at the things ending in 0 and 1 (like ) or things when squared end in 5 and 6 (like , and then )
but honestly I think it would be quicker to just get out a calculator and start cranking them out, especially because we were lucky that it ended in 1.