System of Equations Questions

What is the solution to the following system of linear equations?
$$3x-9y+4z=5$$
$$2x+7y+z=12$$
$$3x-z=0$$

The two equations $3x+6y=15$ and $6x+12y=30$ have

Each of the five persons $A,B,C,D\text{ and }E$ possesses an unequal number (less than $10$) of similar items. $A,B\text{ and }C$ possess $21$ items in all, while $C,D\text{ and }E$ possess $7$ items in all. How many items do $A$ and $B$ possess in all?