Problem returning a Negative Numbers

How can i return a negitive numer ill explain.

box2d wants a -5 for asserting force to the left.

i want to make this happen from an expression, so my express would be.

set velocity of body(??) to -(speed"??")/5

this fails how do i convert the speed from a positive numer to a negative, or am i doing somthing wrong with my expression in the first place.

0-(speed"??")/5
-1*(speed"??")/5

that was so simple realy was'nt it.

thank you MuddyMole, and by all means give me a virtual slap around the head for that.

If you want it for sure to be negative, you can use 0-abs(Speed( "??" ))