To solve these, I like to write it out in expanded form for each rectangle.
Starting from left to right:
(12*7)+(12*5)+(12*7)+(7*5)+(12*5)+(7*5)
It can also be written as:
2(12*7)+2(12*5)+2(7*5)
This equals 358cm^2
It is squared because it is a result of multiplying 2 dimensions: length*width.