(3+5i)^2 = (3+5i)(3+5i)
= 3(3) + 3(5i) + 5i(3) + 5i(5i)
= 9 + 15i + 15i + 25i^2
= 9 + 30i + 25(-1)
= 9 + 30i - 25
= -16 + 30i

Ответ: (-16 + 30i)