help

I guess you mean:

Solve for b1 in:
a=1/2*(b1+b2)

In which case:
a = b1/2 + b2/2
a-b2/2 = b1/2
2*a-b2 = b1

Or, perhaps you mean:

Solve for b1 in:
a=1/(2*(b1+b2))

In which case:
1/a = 2*(b1+b2)
1/(2*a) = b1+b2
1/(2*a) - b2 = b1