Question 607126
That is almost correct, because x=3 is the cutoff point.  But this expression is real when x is less than or equal to 3, not when x is greater than 3.  You can see that by trying a larger number, such as x = 10. Then (15-5x) is negative, and the square root is not real.
On the other hand, when x = 0, (15-5x) = 15, and the square root is real.
So the answer is (-infinity, 3).