According the graph DF is the addition of DE and EF, this can be used to obtain the value of x
[tex]5+(3x+7)=(5x+6)[/tex]solve the equation obtained for x
[tex]\begin{gathered} 12+3x=6+5x \\ 12-6=5x-3x \\ 6=2x \\ x=\frac{6}{2} \\ x=3 \end{gathered}[/tex]if x=3 replace in the equation of DF
[tex]\begin{gathered} DF=5\cdot(3)+6 \\ DF=15+6 \\ DF=21 \end{gathered}[/tex]DF is equal to 21 cm.