You should be able to identify a small 6cm by 6cm square at the bottom right of the figure.

Area of that square = 6cm x 6cm = 36cm²

The remaining area of T = 120cm² - 36cm²
= 84cm²

The remaining area is made up of 2 rectangles. Each of them has a breadth of 6cm.

One of the rectangles's lenght equals the length of S.
The other rectangle has a length that equals the breath of S.


Remaining area of T

= Area of the two rectangles
= 6cm x length of S + 6cm x breadth of S
= 6cm x (length of S + breadth of S)
= 84cm²

length of S + breadth of S = 84cm² ÷ 6cm
= 14cm

Perimeter of S = 2 length of S + 2 breadth of S

= 14cm x 2
= 28cm

Another way to visualise :

You can join those 2 small rectangles to form a larger rectangle, with breadth = 6cm and length = (length of S + breadth of S)