Question 987363: I am trying to factor this equation but my answer is not the same as the answer key. The equation is x^2+4x=21. The answer key shows the answer as {-7,3} IF 21 is not negative, where did the -7 come from. To get positive 4 you subtract positive 3 from positive 7 which when these two are multiplied =21. I am confused.
Answer by Boreal(15235)   (Show Source):
You can put this solution on YOUR website! x^2+4x-21=0, rewriting. The equation has to be in this form =0.
(x+7)(x-3)=0
x= -7, 3
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check
49-28-21=0
9+12-21=0
The 21 is negative, because you have to bring it to the left side. To factor or use the quadratic formula, the equation has to be written in the form Ax^2+Bx+C=0.
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