Can anyone with this? (ignore question b)

-- ' Y ' plus ' Z ' must be equal to ' X ' .

-- A more elegant statement:   

           (The sum of everything flowing into the junction)
is equal to
           (the sum of everything flowing out of the junction).

-- An even more elegant statement:

If water flowing into the junction is positive and
water flowing out of the junction is negative, then

     (The sum of all flow at the junction)  =  Zero.

The reason for all of these statements is the Law of Conservation of Water:

                   Water can't be created or destroyed.

If there are no faucets or leaks at the junction, then the flow in and
the flow out are equal.