f(x) = 7x + 49; g(x) = x + 3

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Stringer

This is not a composite function. If you are asking about
f(g(x)) = 7g(x) + 49 = 7(x + 3) + 49 = 7x + 70 or
g(f(x)) = f(x) + 3 = (7x + 49) + 3 = 7x + 52,
in either case the domain is the entire real numbers--this is the equation of a straight line.

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