Actually u have to first determine which direction ( upward or downward ) to be taken as +ve or -ve .
in ur case u have assumed upward direction as +ve and downward as -ve.
Hence u have to take acceleration due to gravity in negative terms as it is in downward direction . And u know that the slope of v- t graph gives acceleration. Slop is +ve for +ve ( go all the way up) acceleration and -ve (go all the way down) for -ve acceleration .
Hope it helps....!!!!!
