Is a zpak stronger than augmenton 875 mg ?

Nope: Two different types of antibiotics in different classes with different mechanisms of action. In fact, Augmentin (amoxicillin and clavulanate) is a combination of Amoxicillin & clavulanate, neither of which is in the same class as zpak (azithomycin). When considering antibiotics, don't compare strength of one to another. That's like asking whether gasoline, diesel or propane is a better fuel. What you want is the appropriate rx.