When you are estimating, you are simply using numbers that allow for easy computation in your head. For example, when looking at 137,638, I would look to round it somewhere in the range of 140,000. This is because it is closer to 140,000 than it is to 130,000. The second number would round from 52, 091 to approximately 50,000, again because it is closer to 50,000 than it is to 60,000.
Using these numbers, you could estimate the sum of 140,000 + 50,000 to be ABOUT 190,000. For a more accurate answer, you could look at other place values, but these were the first that came to mind. Research has also shown, trust your gut instinct because it's usually correct. Good luck!