We are given function: f(x) = 12x+3.
Generally we start x from 0 and we can take any number for x to get some real number of f(x).
Therefore, domain of the function start from 0.
And we can take any number 0 or greater than 0 to get some value of f(x).
So, we can say that domain of the function is " All real numbers greater than or equal to 0".
Therefore, Prectical domain : All real numbers greater than or equal to 0.