write (a + b)(b – a) as (b + a)(b – a) just to make the difference of squares more visual
so (b + a)(b – a) = b2 – a2 multiply this by –1 and it is [ –1 • (a2 – b2) ]
divide the above by (a2 – b2)
the (a2 – b2) in the numerator and denominator cancel out, leaving you with –1