$$ \begin{aligned} \frac{2}{3}x^2+15x^2 &= 684&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3 \cdot \frac{2}{3}x^2+3\cdot15x^2 &= 3\cdot684&& \text{cancel out the denominators} \\[1 em]2x^2+45x^2 &= 2052&& \text{simplify left side} \\[1 em]47x^2 &= 2052&& \text{move all terms to the left hand side } \\[1 em]47x^2-2052 &= 0&& \\[1 em] \end{aligned} $$
$ 47x^{2}-2052 = 0 $ is a quadratic equation.
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