Data Sufficiency (DS) | Re: Is 3x+7y an integer?

Case 1

(x+y)^3 is an even integer.
Take x=1.6 and y = 0.4 x+y = 2 but 3x+7y = 3*1.6 +7*0.4= 4.8 + 2.8 = 7.6 False
Take x = 2 and y = 4, x + y = 6 3x+7y = 3*2 + 7*4 = 6+28=34 True
Insufficient

Case 2

(x-y)^3 is an even integer

Take x = 5 and y = 3 , x-y =2 , 3x+7y = 3*5 +7*3 = 36 True
Take x = 2.4 y = 0.4 x-y = 2 but 3x+7y = 3*2.4 +7*0.4 = 10 true
But take x = 2*2^1/3 and y = 22^1/3 x-y = 2^1/3 (x-y)^3
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