Group 'em together
a b − a + 1 − b
a b − a = a ( b − 1 )
Notice that there will be a 1 as without it it'll simply be ab
1 − b = 1 ( 1 − b )
Notice that it doesn't match with the upper one... so we'll change the signs
1 ( 1 − b ) = − 1 ( b− 1 )
(try to multiply them now!!
Jot them down in one expression
a ( b − 1 ) − 1 ( b − 1 )
You get!!!!!!
( a − 1 ) ( b − 1 )