If a(x) = 3x + 1 and b(x)=square root of x-4 , what is the domain of (b*a)(x)? ,+infinity) b.(0 , +infinity) c.(1 , +infinity) d.(4 , +infinity)

B. (0, +infinity)

When it is asked to know (b×a)(x), it is being asked to know what are the Y coordinates that are common in both graphs. When it a multiplication think of "and" - what are the Y that are on one graph and simultaneously on the other graph.
Design the graph of both functions on a calculator, or online, wherever. Then, look for when both functions have coordinates. All of them that are common to both graphs are the domain of (b×a)(x).

I'm just guessing, but my answer would c. 0 to 300.