86.345 is the actual sum. If you were to estimate, maybe try saying “85” or “86.3”. I got these number by only adding 34 and 51 and only 34.7 and 51.6. This way your estimate isn’t the exact number, but should be close to it.
34.719+51.626= 86.345
34.7+51.6= 86.3
34+51=85