1) From the chemical equation you obtaing the theoretical molar ratios:
2 mol of M : 3 mol of CaSO4
2) Convert 5.451 g of CaSO4 into number of moles
number of moles = mass in grams / molar mass
molar mass 0f CaSO4 = 136.14g/mol
=> number of moles of CaSO4 = 5.451 g / 136.14 g/mol = 0.0400 mol
3) Make a proportion with the unknown, 0.0400 mol and the theoretical molar ratio:
x / 0.0400 mol of CaSO4 = 2 mol of M / 3 mol of CaSO4
=> x = 0.400 * 2 / 3 mol of M = 0.02667 mol of M
4) Use the mass of 1.2 g and the number of moles to obtain the molar mass of M
molar mass = mass in grams / number of moles
=> molar mass = 1.2 g / 0.02667 moles = 45 g/mol
5) Use a periodic table to find the molar masses of different metals.
Scandium atomic mass is 45 g/mol, so the metal is Scandium.
Answer: Scandium