This is a unit conversion problem. Nm (nanometer) is smaller than mm (millimeter).
1 mm = 1/10³m = 1 x 10⁻³ m
1 nm = 1/10⁹ m = 1 x10⁻⁹ m
1 m = 1 x 10⁹ nm
Hence, 2 mm = 2 x 10⁻³ m
Then we can convert 2 x 10⁻³ m to nm by multiplying with 10⁹.
Then 2 mm = 2 x 10⁻³ m = 2 x 10⁻³ x 10⁹ nm = 2 x 10⁶ nm.
Hence, the thickness of nickel is 2 x 10⁶ nm.