the back part of my cellar where I have my washer and dryer is uneven, and slopes from back to front, over this area it is about 2 inches uneven. Should I use 2x4 or bigger wood to use as a form for my concrete pad? Any other tips?

Would recommend 3'' concrete at back with wire mesh in concrete. Also some pins into existing concrete.

Extend pad enough to stand on when loading washer, about 24''.

Chuck