What is the domain of the function f(x) = 2x + 5?

Here, Function, f(x) = 2x + 5

Let, y = 2x + 5
Subtract both sides by 5, 
y - 5 = 2x

Now,Divide both sides by 2,
y/2 - 5/2 = x

Since,Range is the set of all valid y values, It could be anything here!

In short, Your Answer would be: (-∞, ∞)

Hope this helps!