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It may be

Since it is mental arithmetic and no restrictions.
3, -3, 4i, -4i

since:

a) 3 -3 =0
b) 3 + 4i = (2 + i)(2 + i)
c) 3 - 4i = (2 - i)(2 - i)
d) -3 + 4i = (1 + 2i)(1 + 2i)
e) -3 - 4i = (1 - 2i)(1 - 2i)
f) 4i - 4i = 0

and also:

3 - 3 + 4i - 4i =0

It may be

Since it is mental arithmetic and no restrictions.
$3, -3, 4i, -4i$

since:

a) $3 -3 =0$
b) $3 + 4i = (2 + i)(2 + i)$
c) $3 - 4i = (2 - i)(2 - i)$
d) $-3 + 4i = (1 + 2i)(1 + 2i)$
e) $-3 - 4i = (1 - 2i)(1 - 2i)$
f) $4i - 4i = 0$

and also:

$3 - 3 + 4i - 4i =0$

It may be

Since it is mental arithmetic and no restrictions 3, -3, 4i, -4i since a) 3 -3 =0 b) 3 + 4i = (2 + i)(2 + i) c) 3 - 4i = (2 - i)(2 - i) d) -3 + 4i = (1 + 2i)(1 + 2i) e) -3 - 4i = (1 - 2i)(1 - 2i) f) 4i - 4i = 0 and also 3 - 3 + 4i - 4i =0 (Apologies for the formatting)

It may be

Since it is mental arithmetic and no restrictions.
3, -3, 4i, -4i

since:

a) 3 -3 =0
b) 3 + 4i = (2 + i)(2 + i)
c) 3 - 4i = (2 - i)(2 - i)
d) -3 + 4i = (1 + 2i)(1 + 2i)
e) -3 - 4i = (1 - 2i)(1 - 2i)
f) 4i - 4i = 0

and also:

3 - 3 + 4i - 4i =0

It may be

Since it is mental arithmetic and no restrictions 3, -3, 4i, -4i since a) 3 -3i =0 b) 3 + 4i = (2 + i)(2 + i) c) 3 - 4i = (2 - i)(2 - i) d) -3 + 4i = (1 + 2i)(1 + 2i) e) -3 - 4i = (1 - 2i)(1 - 2i) f) 4i - 4i = 0 and also 3 - 3 + 4i - 4i =0 (Apologies for the formatting)

It may be

Since it is mental arithmetic and no restrictions 3, -3, 4i, -4i since a) 3 -3 =0 b) 3 + 4i = (2 + i)(2 + i) c) 3 - 4i = (2 - i)(2 - i) d) -3 + 4i = (1 + 2i)(1 + 2i) e) -3 - 4i = (1 - 2i)(1 - 2i) f) 4i - 4i = 0 and also 3 - 3 + 4i - 4i =0 (Apologies for the formatting)