Answer :

2a+2b=c make b the subject 
(-2a) from both sides
2b=c-2a 
Divide by 2 both sides 
b=(c-2a)/2
naǫ
[tex]2a+2b=c \\ 2b=c-2a \\ b=\frac{c-2a}{2} = \frac{c}{2} - \frac{2a}{2}=\underline{\underline{\frac{1}{2}c-a}}[/tex]