Answer:
b^2-16 , 16a^2-4
Explanation:
m^2+36 is not a difference its a sum so that is automatically ruled out
3x^2-4, 3 isn't a squared number so it can not be used
b^2-16 , is a difference of squares - (b-4)(b+4)
16a^2-4, is also a difference of squares - (4a-2)(4a+2)