I have to prove that (a - b) x (a + b) = 2(a x b)

I get this:

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

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

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

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

-(a x b)

0 + -b x a + -b x b = a x b

a x b + -b x b = a x b - (a x b)

-a x b

-b x b = 0

and I can't do anything

Thanks for your help