Answer:
(-infinity, infinity), or all real numbers
Explanation:
If you were to look at it graphically, it would be a linear function with a slope of 3 and shifted up 5 units. (i attached a screenshot of what the graph would look like at the bottom :)
since there are no domain restrictions, you're not really going to have any issues or gaps which is when you might need to do something like
(-infinity,4] u(5,inifinity) --> since this isn't the case though, you're totally free to just write all real numbers, because all real numbers in the x-plane (as that is what domain looks at) are included.
When all else fails, (at least for homework), desmos is great for graphing and looking at everything visually
Hope that helps! lmk if you have any questions :)