Represent the lengths of the three sides. They are x, 2x and x+26.
Adding thse together gives us the perimeter, P:
P = x + 2x + x + 26 = 178 feet
Then 4x + 26 = 178; 4x = 152 and x = 38 feet
The side lengths are 38, 76 and 64 feet.
Check by adding these together. The sum must be 178 feet.