Pls solve this question. Thank you

As,  x² -4 is a factor of ax⁴+2x³-3x²+bx-4

therefore, (x² -4) = (x+2) (x-2) is a factor of ax⁴+2x³-3x²+bx-4

now, 

(x+2)  is a factor of ax⁴+2x³-3x²+bx-4

(x-2)  is a factor of ax⁴+2x³-3x²+bx-4

on adding (i) and (ii) , we get,

16a=16

a=1

put in (i), we get,

8-b=16

b=-8

Hope you got it.
Regards!