You mean a single trig function to represent that? I don't think it can be done. However, to answer your comment about the double angle formulas this is what you would get:

f(x) = 3cos(2x) + 4sin(2x) = 3(cos^2(x) - sin^2(x)) + 4*2sin(x)cos(x)

As it happens you can factor this:

f(x) = [3cos(x) - sin(x)][cos(x) - 3sin(x)]

But that doesn't give you the form you want.

If you mean can you get it just in terms of sine or cosine, THAT we can do:

-Dan