If and , then .

Ok this is what I need to prove. I am fairly new to this so i'm not too sure what i have done is right, but here is what i have.

(subtract a)

(multiply by -1)

so (??? not sure after this point ???) . therefor so which equals .

Please if i have don something wrong correct me or if there is a better way of going about this that would be helpful too. Thanks for you help.