The first thing I think when I see this is what is the closest square number that I know close to 355? That would be 400 which is 20². 355 is clearly less than 400 so the next smallest square number would be 19² which is 361. 361 is still bigger than 355 so the next smallest square number would be 18² which is 324.
Now 324 is less than 355 and 361 is bigger than 355 the square root of 355 must lie between the square roots of the two numbers.