How can I get the right notation?(e^3x)

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You may try :

sage: foo=e^3*x
sage: foo
x*e^3
sage: foo.coefficient(x)
e^3
sage: print("%s%s"%(foo.coefficient(x),x))
e^3x

but this is, IMNSHO, chemically pure (analytical quality) foolishness : you're aiming to something (printing æsthetics) that has no algorithmic definition.

A ((very) slightly) little less silly :

sage: R1.<t>=PolynomialRing(SR)
sage: sum([var("p{}".format(u))*t^u for u in range(5)])
p4*t^4 + p3*t^3 + p2*t^2 + p1*t + p0