Solve it without calculus or quadratic

A person starts walking from point $$A$$ with a certain velocity on the highway. Point $$C$$ is in the grassland at a distance $$l$$ from the highway and he wants to get there as soon as possible. However, his speed is $$Q$$ times slower walking on grass than on highway. At what distance $$x$$ $$(AD>x)$$ from point $$D$$ should he change his path so that he reaches point $$C$$ in minimum time.

Assume $$l=9\text{ m}$$, $$Q= 10^{\frac{1}{2}}$$.

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