I need to turn the equation L=12/x + x into a quadratic forumla to solve. i have been given the answers of 0.619m 0r 19.38m if L = 20m

what iv go so far

0= 12/x + x -20 but i dont know how to turn 12/x to a x^2 value

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Multiply the equation by $x$ and you'll get the polynomial you want.
\begin{align} L &= \dfrac{12}{x} + x \\ Lx &= 12 + x^2 & (\text{multiplying both sides by x}) \\ x^2 + 12 - Lx &= 0 \\ \\ \text{For L = 20,} \\ x^2 - 20x + 12 &= 0 \\ \\ \text{Solving using the quadratic formula} \\ x = 0.619 \text{ or } x = 19.380 & \text{as required} \end{align}