Tanka “are mood pieces, usually about love, the shortness of life, the seasons, or sadness. Tanka use strong images and may employ the poetic devices, such as metaphor and personification, that haiku avoid.”*
Japanese poetry like haiku, tankas, chokas, etc. are not really properly matched by English syllables but because it’s easier to specify a form that is based on a specific number of syllables and it is familiar to anyone who has been instructed to write a haiku in school we will retain that model. So… our tanka structure will be 5, 7, 5, 7, 7; with the last line being a repetition of the idea expressed in the line before it (not an actual repition of that line).
*from the same book I referenced last time.