This can be done algebraicly, and without what seems to look like a not understandable copy and pasted answer.
Using some factoring:
6xyz + 30xy + 21xz + 2yz + 105x + 10y + 7z = 812
3x(2yz + 10y + 7z + 35) + (2yz + 10y + 7z) = 812 <------ factor out 3x, and "join like terms"
We see that the difference between the first and the second is "35", so we add 35 to both sides:
3x(2yz + 10y + 7z + 35) + (2yz + 10y + 7z + 35) = 847
We can factor out a 2y and a 7 from the left:
3x(2y + 7)(z + 5) + 1(2y + 7)(z +5) = 847
Now, we can factor out (2y + 7)(z + 5).
(3x + 1)(2y + 7)(z + 5) = 847
The prime factorization of 847 is 7 * 11 * 11, which can be applied to our factored form (they are both factors....get it?):
So:
3x + 1 = 7: x = 2
2y + 7 = 11: y = 2
z + 5 = 11: z = 6
Therefore, x + y + z = 10
:)