Update formula in a subform with a value in the form

Update formula in a subform with a value in the form

Hi,

I am looking to update some values in a subform with changes in the forms... I created the formula that multiplies a value in the form with one in the subform. But if I change the value in the form, the calculation is not redone. 

I am looking something to write in the Action update , but I can't seam to write it correctly

Can somebody help me

Thanks