Verified answer

y(x) = 65.98 ln(x/3.5)

y(x) = 65.98 * [ln(x) - ln(3.5)]

a) y(x) = 65.98 * [ln(6) - ln(3.5)]

= 65.98 * 0.5389965

y(x) = 35.56 ≈ 36 year

1990 + 36 = 2026 => in 2026 the population will be 6 billions

I hope this helps.