She's going to use 2/3 of the frosting(6 cups) to decorate cakes. 'of' means multiplication. So we can multiply to figure it out:
[tex]\sf\dfrac{2}{3}\times 6[/tex]
[tex]\sf\dfrac{12}{3}[/tex]
[tex]\sf\boxed{\sf 4}[/tex]
So Rita will use 4 cups of frosting to decorate cakes.