From the question you can observe that , the number increase by 2 , and the sign is alternating , so number increase by 2 can be represent by 2n , and alternating sign can represent by (-1)^n , combine both u get 2n*(-1)^n , substitute n =1,2,3,4 into it u get -2,+4,-6,+8 right? So add negative sign to it ,becomes -2n*(-1)^n , therefore u will get +2,-4,+6,-8 when substitute n=1,2,3,4