Answer:

Explanation:
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Standard form of a Quadratic:

We have to solve the quadratic by factoring and using the zero product property.
First, convert it to standard form.
Now, find two numbers that add up to -4, but multiply to -21. The numbers that work are -7 and 3. Expand -4a to -7a and 3a.
Factor by grouping
The solutions for a are -3 and 7.