For number seven we know that the square root of 49 is 7 and the square root of 64 is 8. Because 57 is in the middle of these numbers we can assume that the answer is around 7.5 but because it is closer to 64 the proper estimate would be about 7.6. You can check this by plugging the equation into a calculator.
For number eight we know that the square root of 25 is 5. 26 is just over 25 so it's square root must be just over 5. This leads you toward the estimate of 5.1. You can also double check this with a calculator.
