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Well, imagery is a poetic device that uses descriptive language and it appeals to your senses, means the words bring a mental picture of a thing being described. Now, in the poem you need to look for the words that touch your senses. What could be those words? Fire? Yes, fire is imagery as it elicits a feeling of light and heat and also burning, agony and pain.
Now come to symbolism, it is using symbols to represent or signify someone, or something. So, here fire is both imagery and symbol, therefore it looks tricky. However, how is it? Fire is a symbol of desire. Another symbol is ice that symbolizes hate. I hope, now you would have understood. Keep giving your feedback.