equation

1/2c-a

b=1/2c-a

2a + 2b  =  c           Subtract  2a  from both sides of the equation.

2b  =  c - 2a            Divide both sides of the equation by  2  .

b  =  $\frac{c-2a}{2}$

This is the same as..

b  =  $\frac12$c - a            

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