To calculate for the number of moles of SnO2 in the sample, we need the value of the products from the reaction. Since we do not have that data, we can assume that all of the sample is SnO2. Then, we are given the molecular mass of the sample, 151g/mol.
So, the number of moles is 3g / 151 g/mol = 0.0199 moles SnO2