Changing only the drive shows the force-balance arithmetic. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Hold the drag source fixed
The drag source still gives 12 N from the same speed and coefficient.
3⋅4=12
Extra drive leaves net force
Drive 18 N plus the signed drag -12 N leaves net force 6 N.