.
From the condition, you have this equation
    x + (x+400) + (x + 500) = 1500 feet,
where x is the length of the shortest side.
From this equation
    x = 
 = 200  feet.
Thus the sides are  200 ft, 200+400 = 600 ft  and  200+500 = 700 ft.
The triangle inequalities are hold, so such a triangle does exist.
ANSWER.  The sides are  200 ft,  600 ft  and  700 ft.
Solved.