What is the lowest output value approached by the function f(x)=2x−5?

Are you sure you wrote it right?  :-(

The function f(x)=2x-5 is a linear function.

Unless otherwise stated (and that does not seem to be the case) in the real numbers domain, x may assume any value.

The same can be said to y.  

So, the lowest output value approached by this function is -∞

(minus infinity).

:-)

radiative zone

explanation    

radiative zone is the layer of the sun which energy is transferred away from the core by radiation. although, the core is the only part of the sun that produces an appreciable amount of heat through fusion. in fact, 99% of the energy produced by the sun takes place within 24% of the sun's radius.