Using the formula:
z = xy^2

Answer:

6) C

z = xy²

y² = 1.2 x 10¹³
         3 x 10⁻⁶

1.2 x 10¹³ = 3 x 10⁻⁶ (y²)

y² = 0.4 x 10¹⁹

y² = 4 x 10¹⁸

y = 2 x 10⁹