A substance is 89.2% carbon by mass. how much of the substance would be needed to recover 34.6 mol of pure carbon?

1 mole of carbon contains 12 g

Thus, 34.6 moles will contain; 34.6 * 12 = 415.2 g

If a substance contains 89.2 % carbon,

then, (415.2/89.2) * 100 = 465.47 g of the substance will be required to yield 34.6 moles of carbon.