we know that
volume of the box=length*width*height
length=2x+2
width=x-2
height=x
perimeter of the box=2*[length+width]
90=2*[2x+2+x-2]----> 90=2*3x-----> 6x=90----> x=15 cm
so
length=2x+2----> 2*15+2----> 32 cm
width=x-2------> 15-2----> 13 cm
height=x------> 15 cm
volume of the box=32*13*15-----> 6240 cm³
the answer is
6240 cm³