(3 + i)(3 - i)(4 + 5i)
Multiply out the brackets:
(3 + i)(3 - i)(4 + 5i)
= (3 + i)[3x4 + 3x(5i) - ix4 - ix(5i)]
And since, i x i = - 1
So,
= (3 + i)[3x4 + 3x(5i) - ix4 - ix(5i)]
= (3 + i)[12 + 15i - 4i - (-5)]
= (3 + i)[12 + 5 + 11i]
= (3 + i)[17 + 11i]
= 3x17 + 3x(11i) + 17xi + ix(11i)
= 51 + 33i + 17i -11
= 40 + 50i