SOLUTION: Please help me solve this equation{{{a^2 + 4a - 21 = 0}}}

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Question 72822: Please help me solve this equationa%5E2+%2B+4a+-+21+=+0
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
First we need to know what 2 numbers add to 4 and multiply to -21: these two are 7 and -3. So the equation looks like this
a%5E2+%2B+4a+-+21+=+0
%28a%2B7%29%28a-3%29=0The previous equation is factored into this. Now you can solve for a
a%2B7=0
a=-7
a-3=0
a=3
So the solutions are a=-7 and a=3
Check:
Let a=-7
%28-7%29%5E2+%2B+4%28-7%29+-+21+=+0
49-28-21=0
0=0Works
%283%29%5E2+%2B+4%283%29+-+21+=+0
9%2B12-21=0
0=0Works