s^2+16 is prime, you can not factor any further. This is a question meant to trap you into a common mistake - you can easily factor a difference of squares, students commonly try to do the same thing with a sum of squares.
s² + 16 = 0
Use the quadratic formula to factor
   as²+bs+c=0
where
   a=1
   b=0
   c=16
s = [-b ± â(b²-4ac)] / (2a)
   = [-(0) ± â((0)² - 4(1)(16))] / (2·1)
   = [0 ± â(-64)] / 2
   = [0 ± â64â(-1)] / 2
   = [0 ± 8i] / 2
   â 0 ± 4i
   = -4i, 4i