Two ways to do this.
(1) (2x+4)-(3x+6); evaluate the contents of the parentheses after putting x=4:
2x+4=2×4+4=8+4=12; 3x+6=3×4+6=12+6=18;
so (2x+4)-(3x+6)=12-18=-6.
(2) (2x+4)-(3x+6)=2x+4-3x-6=2x-3x+4-6=-x-2.
When x=4, -x-2=-4-2=-6.
Either way is valid.