p = a + b + c
p = 2400
a = longest = 2c - 200
b = middle = a - 200 = 2c - 200 - 200 = 2c - 400
2400 = (2c - 200) + (2c - 400) + c
2400 = 5c - 600
2400 + 600 = 5c
3000 = 5c
3000/5 = c
600 = c <== shortest side
a = 2c - 200.....= 2(600) - 200 = 1200 - 200 = 1000<== longest side
b = 2c - 400...= 2(600) - 400 = 1200 - 400 = 800 <== middle side