Well, for
solving system of equations, we can use either substitution where we plug one
equation into the other, or elimination where we combine the equations.
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Using elimination,
you would to eliminate one variable from both equations, so you automatically would
get one equation with one variable!
Keeping in mind that for a cost C(x) and profit P(x) and revenue R(x), the marginal cost, marginal profit and marginal revenue are respectively dC/dx, dP/dx and dR/dx, then