X=1, x=-7 would be the correct answer.
Step-by-step explanation:

The formula for compound interest is : A = P(1+r/n)^nt
A = 4000 (1+0.021/4)^4x4
A = 4000 (1.021)^16
A = 5,577.91
Sergio's total interest would be : 5,577.91 - 4,000 = $ 1,577.91
hope this helps