Check out the video here:
https://youtu.be/DW6jdZTmPJA

Thanks. May i know why id P+S = Q+R = 1/2 x36? Thanks

In any square or rectangle, the base is length and height is another length or width, their area is ½ of square or ½ of rectangle.

Example, we give length to the square is 4 cm.
Area of Q+R = ½ × 4 × 4 = 8 cm²
Whereas, area of square = 4× 4 =16 cm²
Since Q+R = 8 cm² it is half of square.
Then, Area P+S = 16-8 = 8cm² is half of square too.

Note: Base must be total length of square and height must total length of square too.