How do you solve x^2 +4x=21?

1 Answer
anor277
Mar 26, 2016

By simple factorization. Failing that, by use of the quadratic formula.

Explanation:

We are given that x^2 + 4x =21, or x^2 + 4x - 21 =0. This is a quadratic equation, which is in principle soluble. Factorization presents an easier method.

(x+7)(x-3) = x^2 + 4x - 21 upon expansion.

So, if (x+7)(x-3) = 0, then clearly x has roots of -7 OR +3. Substitution of either of these values for x gives 0. Other posters may give a more systematic method for solution.

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