Question 113511
{{{A=(1/2)(b+B)}}}


First multipy both sides by 2.


{{{2A=2((1/2)(b+B))}}}
{{{2A=b+B}}}


Now add -b to both sides.


{{{2A-b=b-b+B}}}
{{{2A-b=B}}}


Then, just to be neat and so the statement reads naturally, turn it around so that the B is on the left.


{{{B=2A-b}}}


Done.


Hope this helps.
John