Find product of complex number and its Conjugate

Question

3 + i

I don't understand how they got a real number as the answer, the choices are

-112

-2

130

20

thanks.

Answer 1

The conjugate of a + bi is a - bi.

(3 + √11 i)(3 - √11 i) = 3² - (√11 i)² = 9 + 11 = 20