An unknown gas effuses at 0.850 times the effusion rate of nitrogen dioxide, NO2. Estimate the the molar mass of the unknown gas

Graham's Law,

Rate 1 / Rate 2 = √[M2/M1]

Just substitute, remembering M (NO2) = 46 g/mol,

0.850 = √[46/M1];

Solving, M1 = 63.7 g/mole. The other gas could be SO2, which has molecular weight 63.9 g/mole.