Rational numbers are numbers that can be written in the form p/q where p and q are integers and q is not equal to zero. In this case, the square root of 81 is 9. It can be expressed in p/q or rational form as 9/1 so the square root of 81 is a rational number. Likewise the numbers -12, 0, and 12/5 are rational numbers.
The square root of 50 is not rational because it ends in an infinite decimal that cannot be expressed as a fraction. Now if you look at the remaining answer of 6.54 you’ll notice that there is a line above the numbers after the decimal. This line is to indicate that the answer resulted in an infinite decimal. Therefore it is also an irrational number.