you have to number the substituents on one side of the double bond independent of the substitutents on the other side. that is, you'll number priority 1 for the methyl group and priority 2 for the hydrogen on the left side of the double bond... then you'll do it again for the methyl group (#1) and hydrogen (#2) on the right side. then you compare both #1 priority substituents. if they're on the same side, it's Z. if they're on opposite side, they're E.
For disubstituted olefins, it's ok to say that if they're on the same size, it's cis. and if they're on the opposite side, they're trans. cis/trans are really most appropriate for disubstituted olefins. you should change to E/Z for tri and tetrasubstituted olefins.