If -2i is a root, then 2i is also a root. If √10 is a root, then -√10 is also a root. Convert all four of these roots into factors using the factor theorem (which says that if R is a root, then (x - R) is a factor).
(x + 2i) (x - 2i) (x - √10) (x + √10)
Multiply these factors together. (It's easiest to multiply the conjugate pairs together first.):
x^4 - 6x^2 - 40
