Question 648551
I think you may have a typo.  I believe it's  a-(-a+b) = 2a-b  or maybe  a-(b-a) = 2a-b.

Anyway,

a-(b-a) = 2a-b

Distribute

a - b + a = 2a -b

Combine like terms

2a - b = 2a -b  


Or if its

a - (-a+b) = 2a -b 

Distribute

a +a -b = 2a-b

Combine like terms

2a - b = 2a - b