let
x---------> the length side of the box
y---------> the width side of the box
we know that
h=3 in
V=390 in³
volume of the box=x*y*3-------> 390=x*y*3-----> x*y=130------> equation 1
x=3+y------> equation 2
substitute 2 in 1
[3+y]*y=130-----> y²+3y=130------> y²+3y-130=0
using a graph tool------> to resolve the second order equation
see the attached figure
the solution is
y=10 in
x=3+y-----> x=3+10-----> x=13 in
the answer is
the width is 10 in
