Let's say the length of the rectangle is y. Then, we can write an equation for the perimeter of a small rectangle.
(x + y) * 2 = 21
Which can be rewritten as x + y = 10.5
Then, we go around the large rectangle and use x and y to write an expression. Starting with the top left length:
y + x + y + x + y + x + x
4x + 3y
We know that x + y = 10.5, meaning if we multiply, we get 3x + 3y = 31.5
Rewrite 4x + 3y as
3x + 3y + x
Substitute
31.5 + x
That means the perimeter of the large rectangle is 31.5 + x cm.