To find the missing side of the fence, sum all the sides.
Let's say the missing side is x. then,
[tex]\begin{gathered} x+5+8+7+9=35 \\ x+29=35 \end{gathered}[/tex]
To isolate x, subtract 29 from both sides.
[tex]\begin{gathered} x+29-29=35-29 \\ x=6 \end{gathered}[/tex]
Answer: 6 kilometers.
