the longest side = x
the second side is half the longest side = (1/2)x
the third side is 12 feet less than the longest side = x-12
The perimeter (the sum of all sides) is 53.
\(x+\frac{1}{2}x+x-12=53 \\
x+\frac{1}{2}x+x=65 \\
2\frac{1}{2}x=65 \\
\frac{5}{2}x=65 \\
x=65 \times \frac{2}{5} \\
x=13 \times 2 \\
x=\boxed{26} \\ \\
\frac{1}{2}x=\frac{1}{2} \times 26=\boxed{13} \\ \\
x-12=26-12=\boxed{14}\)
The lengths of the sides are 13, 14 and 26 feet.
