b=![\sqrt{21}](/tpl/images/0519/0792/5c3d0.png)
Step-by-step explanation:
Considering the given figure the sides 'b' , '2' and '5' are making the sides of a right angled triangle in the top right corner of the square, where
Hypotenuse=5  , Perpendicular=b and  Base=2
Using Pythagoras Theorem:
        ![(Hypotenuse)^2=(Perpendicular)^2+(Base)^2\\](/tpl/images/0519/0792/fdf7b.png)
    Implementing the values in the formula:
           ![5^2= b^2+2^2](/tpl/images/0519/0792/d4510.png)
           ![b^2=5^2-2^2\\\\b^2=25-4\\\\b^2=21\\\\b=\sqrt{21}](/tpl/images/0519/0792/09122.png)
So, the value of 'b' is
which is same for the the given 'b' in the figure.
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