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\begin{aligned} \frac{D G}{E G} &=\frac{G C}{G F}=\frac{D C}{E F} \\ \frac{D C}{G C} &=\frac{E F}{G F} \\ \frac{2}{G C} &=\frac{3.5}{2.5+G C} \\ 5+2 G C &=3.5 G C \\ 5 &=1.5 G C \\ G C & \approx 3.33 \end{aligned}