Answer:
c = 2
Step-by-step explanation:
Substitute x = 1 + i into the equation
(1 + i)² - 2(1 + i) + c = 0 ← distribute left side
1 + 2i + i² - 2 - 2i + c = 0 ( note that i² = - 1 )
1 + 2i - 1 - 2 - 2i + c = 0 ← collect like terms on left
- 2 + c = 0 ( add 2 to both sides )
c = 2
Whenever you multiply by a negative number.
Hopefully that helps you a little.