Z = Ma = ba

You made a typo. One of those = signs should either 
be a + and a -.  Here are all possibilities:


1.     Z+Ma = ba, solve for a
      Ma-Ba = -Z
     a(M-B) = -Z
          a = -Z/(M-B)

2.        Z = Ma+ba, solve for a
      Ma+ba = Z
     a(M+b) = Z
          a = Z/(M+b) 

3.     Z-Ma = ba, solve for a
          Z = Ma+ba
      Ma+ba = Z
     a(M+b) = Z
          a = Z/(M+b)

4.       Z = Ma-ba, solve for a
      Ma-ba = Z
     a(M-b) = Z
          a = Z/(M-b)

Edwin