So 2 dozen cupcakes = 24 cupcakes
If each cupcake gets 3 walnuts then you would multiply 3x24 to get the number of walnuts needed to decorate them.
3x24=72 walnuts are needed to decorate 2 dozen cupcakes.
Since Henry only has 56 walnuts then he does not have enough to decorate 2 dozen cupcakes