(1 + 2i)z = 5
Multiply both sides of the equation by (1 - 2i) ,
This will give,
(1 + 2i)(1 - 2i)z = 5(1 - 2i)
(1 - 2i + 2i - 4i²)z = 5 - 10i
Now since, i² = -1
This becomes,
(1 - 4(- 1))z = 5 - 10i
(1 + 4)z = 5 - 10i
5z = 5 - 10i
Now divide both sides of the equation by 5,
z = 1 - 2i ,
which is the answer you wanted to find. If you don't understand why or how I did this then please feel free to ask .