To find the molar mass of CaSO4, you have to first find the molar mass of each constituents elements and them add the masses together.
Atomic Mass of Ca = 40.08
Atomic Mass of S = 32.065
Atomic Mass of O = 16.00. Note that, there are four atoms of oxygen.
40.08 + 32.065 + [16.00* 4] = 136.145
Rounded to the nearest gram, the molar mass of CaSO4 is 136.