Question 1193619
DE is half of the hypotenuse or 7 sqrt (3). It is because it is a 30-60-90 right triangle, which is the only kind where one of the angles is half the other. 
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DE is 7*1.73= 12.21
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DF is the short leg * sqrt (3)=21
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it is a 7 sqrt(3)//21//14 sqrt(3) right triangle or squaring each, 147+441=196*3=588