We have the expression:

We factor as follows:

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We know that the expression when factored is the product of two smaller expressions that give as solution the original expression. We also know that in each expression s is present and in each expression, there is a compliment, so they have the form:

We concentrate in the "extremes" of the trinomial (these being 3s^2 and 7).
We know that each is the product of two different values.
So 3s^2 is the product of as*cs, and we can rapidly see that as = s and