You cannot solve one equation in three unknowns. You need three independent equations.
12 + 5 equals...
x=4,y=9,z=5
5
depends what 'a' =
you do 3 plus 5 first then you get 8 you have to find out what minus 7 equals 8
-2 plus 5 equals +3
12 + 5 equals...
A is 14
x=4,y=9,z=5
3
3
5
x= -.5
depends what 'a' =
The equation does not have a solution.
minus 5
x=7