.
x + y + z = 55, (1)
x - z = 51, (2)
y + z = 12. (3)
where x is the largest number and z is the smallest one.
Now, distract (3) from (1). You will get x = 55 - 12 = 43.
Substitute it into (2), and you will find z = 43 - 51 = -8.
Substitute z = -8 into (3), and you will find y = 12 + 8 = 20.
Answer. x = 43, y = 20, z= -8.