What should you do if your estimate is a lot different from your exact answer

You should work your exact answer again, extra carefully, and maybe
a slightly different way if one is possible.  You should also take your
answer and 'check' it ... write it back into the original problem, and
see if the result is correct.

When your estimate is a lot different from your exact answer, it means
that either the estimate or the exact answer is way wrong, and there's
even a good chance that they're both wrong.  So you have to repeat
something, and find a mistake somewhere.

If you repeat your estimate first and don't find the mistake there, then
you'll have to repeat the exact answer.  And if you DO find the mistake
in the estimate, you'll still probably want to repeat the exact answer just
to be sure.  Either way, if you decide to repeat the estimate first, then
whether or not you find a mistake there, you'll repeat the exact answer too.

That's why I suggest repeating the exact answer first. 
You're going to repeat it anyway, and if you find the mistake THERE,
you can forget about repeating the estimate, and save yourself some
time.