ANSWER
DE = 7.9
EXPLANATION
As shown in the diagram, these two triangles are similar by the AA postulate. The corresponding sides are: AC-DF, AB-DE, BC-EF.
In similar triangles, the ratio between corresponding sides must be constant:
[tex]\frac{AB}{DE}=\frac{BC}{EF}[/tex]
Replacing with the measures we know we can solve for DE:
[tex]\begin{gathered} \frac{11}{DE}=\frac{14}{10} \\ DE=11\cdot\frac{10}{14} \\ DE\approx7.9 \end{gathered}[/tex]
