Question 120875
Very difficult to tell whether you have the first step correct or not, because you didn't specify which part of the trapezoid was labeled x.  Since the area of a trapezoid is given by {{{A=((a+b)h)/2}}} and you started with {{{480 = x^2 + 2x}}}, I'm going to assume that the height is x, and the two bases are 2x and 4 respectively.


{{{A=((2x+4)x)/2}}}
{{{A=(x^2+2x)}}}, and since the area is 480,


{{{x^2+2x=480}}}


Assuming that is correct, on to the next step.  Here you have an error.


{{{x^2+2x=480}}}


Add -480 to both sides:


{{{x^2+2x-480=0}}}


This does not factor nicely.  Use the quadratic formula.


{{{x = (-2 +- sqrt( 2^2-4*1*(-480)))/(2) }}} 
{{{x = (-2 +- sqrt( 1924))/2}}}
{{{x = (-2 +- 2*sqrt(481))/2}}}


This ugliness leads me to believe that the initial setup is not correct.  Describe the figure in more detail.