Slant rhyme is when the words do not have perfect rhyme. They could have a similar sound, but it's not identical. Sometimes a reader must alter the pronunciation of the words slightly to create the similar sound. In the poem, fro has a long /o/ sound and through has the sound /oo/. These are not identical, but can be similar if pronounced a certain way.
I would say FRO and THROUGH, because through is said differently then when you're rhyming with FRO, if I could say it outlaid it would make more sense :P but I hope this helps!
