The lines in iambic trimeter are:
2. That’s newly sprung in June:
and
4. So deep in luve am I:
Both those lines present a pattern of three iambic units, each one of two feet, which is the definition of iambic trimeter, as the question states: an unstressed syllable followed by a stressed syllable; pattern which repeats itself three times per line.
The longer lines of the poem are in iambic tetramete instead.
The answers are two and four. I hope you get it right!