For the square roots, the radicand cannot be negative, or you'll end up with an imaginary number.
So, set the radicand ≥ 0 and solve for the value of the variable, observing all the rules of inequalities.
For part b, as an example, you have the sqrt(5-x) in the numerator. Set this: 5-x ≥ 0, therefore, 5 ≥ x.
Then, write in interval notation. (-∞,5].
Check to make sure your denominator will not equal zero at these values.