Maclaurin Limit, please help

$\displaystyle

x ( (x^3 + 3x^2)^{1/3} + (x^2 + 2x - 3)^{1/2} )

$

So, I start off with breaking an x out from both parts of the equations and get:

$\displaystyle

x^2 \cdot ( (1 + 3/x)^{1/3} - (1 + 2/x - 3/x^{1/2} )^{1/2} )

$

Here comes the funky part - I try making two variable changes at the same time, refering to a standard maclaurin polynomial and get:

$\displaystyle

x^2 \cdot ( (1 + u)^{1/3} - (1 + t)^{1/2} ) =

x^2 \cdot ( 1 + u/3 - 1 + t/2 ) =

x^2 \cdot ( 1/x - 1/x + 3/2x^2 ) =

3/2

$

The answer should be 1, so, I assume I'm doing something wrong when I switch variables, any hint on what's going wrong?