functions

Question

f(x)=rad. x-9

write the intervals of the functions. How do I put the set of numbers in the form of intervals such as infinity.

I solved it and got the answer as this:
x < or = 9
(infinity,9]
Please help me, I am stuck.

Answer 1

(infinity,9]

means the interval's lower limit is at but excludes ∞. The exclusion is signified by the left parenthesis "(".
The upper limit is 9, and 9 is included, as indicated by the square bracker "]".

I do not quite understand the meaning of
f(x)=rad. x-9
so I cannot comment on whether the answer is correct.