x+y+z=78
y =2z
z=x+6
We can express x and y from 2nd and 3d equations, and substitute their values into the 1st.
y = 2z and x= z-6
x+y+z=78
z-6+2z+z=78
4z -6 =78
4z= 84
z=21
y =2z , y=2*21, y=42
x= z-6, x= 21-6, x=15
Answer: x=15, y=42, z=21.
Check:
15+42+21 = 78
78=78
