3.0ft. The lenght of the third side of a triangle is 3.0ft.
The perimeter of a right triangle is the sum of the three sides P = a + b + c, where a, b, and c are the sides of the triangle.
If the perimeter and two sides of the triangle are known, it is possible to calculate the length of the third side.
P = a + b + c -------------> c = P - (a + b)
With P = 7.2ft, a = 1.8ft, and b = 2.4ft
c = 7.2ft - (1.8ft + 2.4ft)
c = 7.2ft - 4.2ft
c = 3.0ft