Multiply $$3\sqrt{28}$$ by $$2\sqrt{7}$$.
$$3\sqrt{28}\times 2\sqrt{7}$$
It can also be written as
$$=3\times 2\times \sqrt{28}\times \sqrt{7}$$
So we get
$$=6\times \sqrt{28\times 7}$$
By multiplication
$$=6\times \sqrt{2\times 2\times 7\times 7}$$
$$=(6\times 2\times 7)$$
$$3\sqrt{28}\times 2\sqrt{7}$$$$=84$$.