how can I understand that (move to "a" to "b"à

In C3 you would simply use the tween behaviour with a linear ease:

construct.net/en/make-games/manuals/construct-3/behavior-reference/tween

but scirra don't tell about behaviours

to move at a constant speed, don't use the object's current location as one of the lerp parameters.

and you will need a variable to keep track of how far it has moved (from 0 to 100%)

so, if you have an object ShopP1, and want to move if from 200 to 0 in one second you would use:

Global number PercentDone = 0

PercentDone < 1

- System Add dt to PercentDone

- ShopP1 set X to lerp(200,0,PercentDone)

thank you ,it's a good example what i seach.