Automatically, you can see that :
1. (h+3i)^2 = (h+3i)(h+3i) - don't bother expanding there is no need
2. then you have - 49j^2 . You take the square root of 49 which is 7. Therefore, this is equal to : - (7j)^2 .
The negative sign is really important.
What happens is you end up with an...