Find two complex number roots for the equation x^2-4x+21=0

Not sure I know what this means, could someone explain it to me.

I tried to do this using synthetic division? But couldn't find the zero...

7| 1 -4 21

1 7 21

1 3 42

-3| 1 -4 21

1 -3 21

1 -7 42

I think i should have gotten a -21 resulting in a 0

For any help:

Thanks!