Bring 4 cups water to a boil with 3/4 cup sugar; stir to dissolve. Let cool 5 minutes. Add 8 green tea bags and steep 5 minutes. Remove the tea bags and gently squeeze out the liquid; pour the tea into a pitcher. Stir in 3 cups cold water and 1/2 cup fresh lemon juice (from 3 lemons). Chill. Serve over ice with lemon slices.