(a-2b-4)²+(2b+c)²+|a-4b+c|=0,
(a-2b-4)^2=0
a-2b-4=0
a=2b+4
(2b+c)=0
c=-2b
|a-4b+c|=0,
a-4b+c=0
2b+4-4b-2b=0
-4b+4=0
b=1
c=-2b=-2*1=-2
a=2b+4=2*1+4=6
3a+b-4c=3*6+1-4*(-2)=19+8=27