The little symbol right there (√) denotes the square root of the number thats given after the symbol. So √36 simply means the square root of 36. The square of a number is when the number is multiplied by itself resulting in a new number. So, the square root of the new number is simply the number which when multiplied by itself, gave the new number.
We know, => 6 * 6 = 36
Here, 6 is multiplied by itself to give 36. Therefore, the square root of 36 is nothing but 6 itself.
Here, L = √36 cm = 6 cm
You can now use this value to find the area of the given Figure.