perimeter = 40 feet
the perimeter is the sum of the lengths of the sides
evaluating each side for x = 4
3x - 4 = (3 × 4) - 4 = 12 - 4 = 8
x² - 1 = 4² - 1 = 16 - 1 = 15
2x² - 15 = 2(4)² - 15 = 32 - 15 = 17
the sides have lengths of 8, 15 and 17
perimeter = 8 + 15 + 17 = 40 feet