45° + 90° +gama/2 = pi

135° + gama/2 = 180 °

gama/2=45°

gama= 90°

and 45° + 90° + alfa = 180°

so alfa = 45°

sin(45°)= b/4

and we know sin(45 °)=sqrt(2)/2

sqrt(2)/2=b/4

b=2sqrt(2) = 3

Sorry but the true answer is

true answer.jpg

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