Whisk flour, baking soda, baking powder, cinnamon, salt, and other spices you like in a big bowl. Set the mix aside.
Grab another bowl and whisk your oil, eggs, granulated sugar, brown sugar, and vanilla extract until combined. Then, pour that into your dry ingredients (from Step 2) and whisk. Fold in your pineapple, grated carrots, and walnuts.
Spread the cake batter into your 13 x 9 pan and bake for 45 to 55 minutes (give or take, so keep an eye on it). Your cake is ready when a small toothpick poked in the middle comes out mostly clean. Cover the cake loosely with foil if it starts to brown too much on top.
Let it cool on a wire rack once it's out of the oven. You might stick it in the fridge after about 45 minutes to speed things up.