[texhax] standard answer of a math paper

[Li Weihua]

I am giving out a standard answer of a math paper, and I want to get the
following view:

Prove:                
              a^2-b^2=0....................(3point)
              (a+b)*(a-b)=0................(5 point)
              a=-b or a=b..................(2 point)

where the equation stays in the middle of the line, and the score of one
step stays in the right of the line, like an equation num,but between
the num and equation fill with point instead.

How can I get that? 
Thanks.

Li Weihuastandard answer of a math paper