Wee you find the area off all the sides and add em all up. First, for the big side to find the area is lw so 13 * 7= 91mm and there are 4 of those sides so 91*4 = 364mm. Next, for the small sides, lw is 8*7=56 and there are 2 so 50*2 = 112mm. And finally you add them together 112+364= 476mm