Question 80479
(y-3)(y+7)=0
y^2-3y+7y-21=0
y^2+4y-21=0
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one way to check his is to use the quadratic equation ay^2+by+c:
{{{y = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
then you'll see that: 
y-3=0
y=3 
&
y+7=0
y=-7