A tennis ball weights about 56.7 grams. About how many ounces would a dozen tennis balls weight?

Nothing "weighs" 56.7 grams. "Gram" is a unit of mass, not force or weight.

On Earth, 28.35 grams of mass weigh about 1 ounce.

1 dozen of anything = 12 of them

If each tennis ball has 56.7 grams of mass, then a dozen of them

have

(12) x (56.7) = 680.4 grams of mass.

Their weight is (680.4 grams) x (1 ounce / 28.35 grams) =

(680.4 / 28.35) = 24 ounces.

That's 1.5 pounds.