Classic! I assume you have one of those handy-dandy reference tables, but if not look up the periodic table. The formula mass is the total mass of all the atoms in the equation, in other words it's all of the atomic masses added up together. So let's begin.
There are 2 aluminum atoms
Al- 2 atoms*27 <--- 27 is the estimated atomic mass number
S- 3 atoms*32
O-12 atoms*16
multiply that out...
Al- 54 grams +
S- 96 grams +
O- 192 grams+ now lets add everything together
__________
342 grams of Al2(SO4)3. Pretty cool?