Which part of a lyric poem is most helpful for finding the poems theme?

In this excerpt from robert burns’s poem “a red, red rose,” which two lines are written in iambic trimeter? (an iambic trimeter pattern contains an unstressed syllable followed by a stressed syllable. the pattern repeats three times per line.) o my luve’s like a red, red rose that’s newly sprung in june: o my luve’s like the melodie that’s sweetly play’d in tune! as fair art thou, my bonnie lass, so deep in luve am i: and i will luve thee still, my dear, till a’ the seas gang dry: