Question 92311
Remember 

{{{A=L*W}}}


{{{3x^2+5x=(x)*W}}} plug in the given information


{{{(3x^2+5x)/x=W}}} Divide both sides by x


{{{3x^2/x+5x/x=W}}} Split up the fraction


{{{3x+5=W}}} Divide each term by x


So the width is 


{{{W=3x+5}}} 



Check:


{{{A=x(3x+5)}}} Plug in L=x and W=3x+5


{{{A=3x^2+5x}}} Distribute. So this verifies our answer