It would help both you and whoever is marking your work to include acceleration arrows on your diagram. Then, if acceleration comes out as negative, you will know that the system is accelerating in the opposite sense to your acceleration arrows. The most crucial thing is to be consistent either side of the pulley. Thus if the 3kg mass is shown as accelerating upwards, the 7kg weight must be shown as accelerating downwards (because the string remains taut). Your working is consistent with the 3kg mass accelerating downwards and the 7kg weight accelerating upwards, which is OK as long as you know you're doing this. Hence you arrived at a negative acceleration.