On the Slider Component itself in the expression field type "-(Slider)". You can negate any input slot on another component as well as long as the name of the slot is in the Brackets.
It is the same like changing the input for a component looking for an angle from radians to degrees. Say the slots name is "A", then in the expression field type "rad(A)".
If you need to reverse the direction of a line there is a flip component under CURVE>UTILITY>FLIP. If you would like to find the reciprocal of a slider value or make it negative, use the expression line within the operation.