7x + 10 - ( x+ 7 ) =
there is an invisable 1 between the minus sign and the backet.
7x + 10 - 1( x+ 7 ) =
Now the minus sign belongs to the 1
-1(x+7)=-1*x+-1*7 = -x-7
so
7x + 10 - ( x+ 7 )
=7x+10 -x-7
=7x-x +10-7
=6x +3
