How to get the lateral area of a rectangular prism? plz explain

If by lateral area you mean the total surface of the rectangle :

each side appears twice so the total area A will be

A=2(length*width+width*height+height*length)=16*8+8*4+4*16
A=224

If you meant lateral area as in the total area minus the top and bottom base:
A=2(width*height+height*length)=8*4+4*16=96