When finding the domain of a function, you want to exclude those values of x which cause the function to be undefined (division by zero) or have complex values (an even root of a negative value). There are also restrictions on logarithmic and inverse trigonometric functions as well.
Polynomial functions such as the first two, have no restrictions on the domain as there is no division or radicals. For the remaining three, set the expressions under the radicals greater than or equal to zero, and solve for x. This will tell you the valid values for x, which is the domain of the function. What do you find?