F(x)=x-3 has all real numbers as the domain. g(y)=4 has all real numbers as the domain. The combined function G(F(x)) = 4 (Substitute x-3 into G(y)=4) for y) has no restrictions.
The domain of the combined function is all real numbers.
Hope the solution helped. Sometimes you need more than a solution. Contact fcabanski@hotmail.com for online, private tutoring, or personalized problem solving (quick for groups of problems.)