Sole the equation for the indicated variable.

zm = a+b/a, for a

zm = a+b/a, for a

Switch sides to have a on left side

a+b/a=zm

Now multiply both sides by a

A+b = zm (a)

A+b = ama

Now subtract azm both sides

A+b-azm=0

Subtract b both sides

A-azm = - b

Factor out a from ask

A (1 - zm) = - b

Divide each term by 1 - zm to get

A = - b/1-zm