You get area when you multiply the 2 sides together
the sides are 8ft and (2+x)ft
when you multiply them together, it looks like
8(2+x) ft^2
so distribute the 8
You get perimeter when you add up all the sides together
the sides 8 and (2+x) are given, but remember the opposite sides are also 8 and (2+x)
when you add up all the sides, you get
8+(2+x)+8+(2+x)