Question 766217
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{{{3*x^2+21*x=0}}}

Factoring out the common element of 3*x,

{{{3*x*(x+7) = 0}}}

So, 3*x = 0 or x + 7 = 0

i.e. x = 0 or x = -7 are the two roots of the quadratic.

Hope this helps :)
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