Heat oven to 350°.
Grease and flour oblong pan, 13 X 9 X 2 inches.
Prepare cake mix as directed on package except stir soda into dry mix; use 1 cup plus 4 teaspoons water and add banana.
Fold in nuts.
Pour batter into pan.
Bake until wooden pick inserted in center comes out clean, 35 to 40 minutes.