It is best to establish a routine and follow it each time you approach a sum.
1. Look at the number on the top row. Say aloud: "Eighty six".
2 Look at the number on the bottom row. Say aloud: "Fifty seven".
3. Say aloud: "Eighty six take away fifty seven".
4. In your head, do a quick estimate of what the answer will be, like this: round 86 to 90, then round 57 to 60. Subtract 60 from 90; that's 30. So the correct answer should be around 30.
5. Look at the units column. Can you take 7 from 6? No. So you must regroup eighty six.
At the moment, it's written as 8 tens and 6 units. Change it to 7 tens and 16 units.
6. Now take 7 units from 16 units, and take 5 tens from 7 tens.
8. Say "Eighty six take away fifty seven equals twenty nine."
9. Think back to your estimate, which was 30... very near to 29.
In this second example, you can follow the same routine as above.
When it comes to rounding 176, look at the tens and units. Round 76 to 80, and say "One hundred and 180". Round 149 to 150. Now it's easy to take 150 from 180.
The correct answer should be around 30.
Note: When subtracting, always work on the units column first, then work on the tens column and finish with the hundreds column.