Tilly and her mom made a rectangular dog run. They used 26 one-yard long pieces of fencing. They put 6 pieces of fencing on one side. How many pieces did they put on the next side?

Perimeter of a rectangle equals length + length + width+ width wherein both length are equal and both widths are equal
therefore if one length takes 6 pieces that means the next length is also 6 pieces
26 - (6 + 6) = 14 pieces
next all you need to do is divide 14 by 2 to get the amount of pieces for one of the other two sides. (but if you get the amount of pieces needed for one side it will also be the same for the other since they would be equal)
14÷2 = 7
they would use 7 pieces on the other side

Set up a system of equations