Let the first number be x, the second be y, and the third be z.
x + y + z = 102
The third number is 2 times the second, so z = 2y
The first number is 6 more than the second, so x = y + 6
We replace each letter by it's y value:
y + 6 + y + 2y = 102
4y + 6 = 102
4y = 96
y = 24
As we stated before, z = 2y = 2*24 = 48
And x = y + 6 = 24 + 6 = 30
So, we get x = 30, y = 24, z = 48
x + y + z = 102
30 + 24 + 48 = 102
Hope this Helps! :D