Question 1196335
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It is  BECAUSE  if you substitute  x= -4  into left side expression,


you will get the number under square root    (-4)^2 +5*(-4) = 16 - 20 = -4.


But negative numbers are not allowed under square root - - - THEREFORE,  we  REJECT   x= - 4.  


The same is with the right side expression:  it does not allow  x= -4.


So,  x= -4  is an extraneous solution.



Hope,  everything is clear now.