Answer is: there are 3·10²³ atoms of carbon-12.
m(C) = 6.00 g; mass of carbon.
M(C) = 12 g/mol.
n(C) = m(C) ÷ M(C).
n(C) = 6 g ÷ 12 g/mol.
n(C) = 0.5 mol.
N(C) = Na · n(C).
n(C) = 6·10²³ 1/mol · 0.5 mol.
n(C) = 3·10²³.
n - amount of substance.
M - molar mass.
Na - Avogadro number.
